Source file src/time/time.go

Documentation: time

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package time provides functionality for measuring and displaying time.
     6  //
     7  // The calendrical calculations always assume a Gregorian calendar, with
     8  // no leap seconds.
     9  //
    10  // Monotonic Clocks
    11  //
    12  // Operating systems provide both a “wall clock,” which is subject to
    13  // changes for clock synchronization, and a “monotonic clock,” which is
    14  // not. The general rule is that the wall clock is for telling time and
    15  // the monotonic clock is for measuring time. Rather than split the API,
    16  // in this package the Time returned by time.Now contains both a wall
    17  // clock reading and a monotonic clock reading; later time-telling
    18  // operations use the wall clock reading, but later time-measuring
    19  // operations, specifically comparisons and subtractions, use the
    20  // monotonic clock reading.
    21  //
    22  // For example, this code always computes a positive elapsed time of
    23  // approximately 20 milliseconds, even if the wall clock is changed during
    24  // the operation being timed:
    25  //
    26  //	start := time.Now()
    27  //	... operation that takes 20 milliseconds ...
    28  //	t := time.Now()
    29  //	elapsed := t.Sub(start)
    30  //
    31  // Other idioms, such as time.Since(start), time.Until(deadline), and
    32  // time.Now().Before(deadline), are similarly robust against wall clock
    33  // resets.
    34  //
    35  // The rest of this section gives the precise details of how operations
    36  // use monotonic clocks, but understanding those details is not required
    37  // to use this package.
    38  //
    39  // The Time returned by time.Now contains a monotonic clock reading.
    40  // If Time t has a monotonic clock reading, t.Add adds the same duration to
    41  // both the wall clock and monotonic clock readings to compute the result.
    42  // Because t.AddDate(y, m, d), t.Round(d), and t.Truncate(d) are wall time
    43  // computations, they always strip any monotonic clock reading from their results.
    44  // Because t.In, t.Local, and t.UTC are used for their effect on the interpretation
    45  // of the wall time, they also strip any monotonic clock reading from their results.
    46  // The canonical way to strip a monotonic clock reading is to use t = t.Round(0).
    47  //
    48  // If Times t and u both contain monotonic clock readings, the operations
    49  // t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out
    50  // using the monotonic clock readings alone, ignoring the wall clock
    51  // readings. If either t or u contains no monotonic clock reading, these
    52  // operations fall back to using the wall clock readings.
    53  //
    54  // On some systems the monotonic clock will stop if the computer goes to sleep.
    55  // On such a system, t.Sub(u) may not accurately reflect the actual
    56  // time that passed between t and u.
    57  //
    58  // Because the monotonic clock reading has no meaning outside
    59  // the current process, the serialized forms generated by t.GobEncode,
    60  // t.MarshalBinary, t.MarshalJSON, and t.MarshalText omit the monotonic
    61  // clock reading, and t.Format provides no format for it. Similarly, the
    62  // constructors time.Date, time.Parse, time.ParseInLocation, and time.Unix,
    63  // as well as the unmarshalers t.GobDecode, t.UnmarshalBinary.
    64  // t.UnmarshalJSON, and t.UnmarshalText always create times with
    65  // no monotonic clock reading.
    66  //
    67  // Note that the Go == operator compares not just the time instant but
    68  // also the Location and the monotonic clock reading. See the
    69  // documentation for the Time type for a discussion of equality
    70  // testing for Time values.
    71  //
    72  // For debugging, the result of t.String does include the monotonic
    73  // clock reading if present. If t != u because of different monotonic clock readings,
    74  // that difference will be visible when printing t.String() and u.String().
    75  //
    76  package time
    77  
    78  import (
    79  	"errors"
    80  	_ "unsafe" // for go:linkname
    81  )
    82  
    83  // A Time represents an instant in time with nanosecond precision.
    84  //
    85  // Programs using times should typically store and pass them as values,
    86  // not pointers. That is, time variables and struct fields should be of
    87  // type time.Time, not *time.Time.
    88  //
    89  // A Time value can be used by multiple goroutines simultaneously except
    90  // that the methods GobDecode, UnmarshalBinary, UnmarshalJSON and
    91  // UnmarshalText are not concurrency-safe.
    92  //
    93  // Time instants can be compared using the Before, After, and Equal methods.
    94  // The Sub method subtracts two instants, producing a Duration.
    95  // The Add method adds a Time and a Duration, producing a Time.
    96  //
    97  // The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
    98  // As this time is unlikely to come up in practice, the IsZero method gives
    99  // a simple way of detecting a time that has not been initialized explicitly.
   100  //
   101  // Each Time has associated with it a Location, consulted when computing the
   102  // presentation form of the time, such as in the Format, Hour, and Year methods.
   103  // The methods Local, UTC, and In return a Time with a specific location.
   104  // Changing the location in this way changes only the presentation; it does not
   105  // change the instant in time being denoted and therefore does not affect the
   106  // computations described in earlier paragraphs.
   107  //
   108  // Representations of a Time value saved by the GobEncode, MarshalBinary,
   109  // MarshalJSON, and MarshalText methods store the Time.Location's offset, but not
   110  // the location name. They therefore lose information about Daylight Saving Time.
   111  //
   112  // In addition to the required “wall clock” reading, a Time may contain an optional
   113  // reading of the current process's monotonic clock, to provide additional precision
   114  // for comparison or subtraction.
   115  // See the “Monotonic Clocks” section in the package documentation for details.
   116  //
   117  // Note that the Go == operator compares not just the time instant but also the
   118  // Location and the monotonic clock reading. Therefore, Time values should not
   119  // be used as map or database keys without first guaranteeing that the
   120  // identical Location has been set for all values, which can be achieved
   121  // through use of the UTC or Local method, and that the monotonic clock reading
   122  // has been stripped by setting t = t.Round(0). In general, prefer t.Equal(u)
   123  // to t == u, since t.Equal uses the most accurate comparison available and
   124  // correctly handles the case when only one of its arguments has a monotonic
   125  // clock reading.
   126  //
   127  type Time struct {
   128  	// wall and ext encode the wall time seconds, wall time nanoseconds,
   129  	// and optional monotonic clock reading in nanoseconds.
   130  	//
   131  	// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
   132  	// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
   133  	// The nanoseconds field is in the range [0, 999999999].
   134  	// If the hasMonotonic bit is 0, then the 33-bit field must be zero
   135  	// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
   136  	// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
   137  	// unsigned wall seconds since Jan 1 year 1885, and ext holds a
   138  	// signed 64-bit monotonic clock reading, nanoseconds since process start.
   139  	wall uint64
   140  	ext  int64
   141  
   142  	// loc specifies the Location that should be used to
   143  	// determine the minute, hour, month, day, and year
   144  	// that correspond to this Time.
   145  	// The nil location means UTC.
   146  	// All UTC times are represented with loc==nil, never loc==&utcLoc.
   147  	loc *Location
   148  }
   149  
   150  const (
   151  	hasMonotonic = 1 << 63
   152  	maxWall      = wallToInternal + (1<<33 - 1) // year 2157
   153  	minWall      = wallToInternal               // year 1885
   154  	nsecMask     = 1<<30 - 1
   155  	nsecShift    = 30
   156  )
   157  
   158  // These helpers for manipulating the wall and monotonic clock readings
   159  // take pointer receivers, even when they don't modify the time,
   160  // to make them cheaper to call.
   161  
   162  // nsec returns the time's nanoseconds.
   163  func (t *Time) nsec() int32 {
   164  	return int32(t.wall & nsecMask)
   165  }
   166  
   167  // sec returns the time's seconds since Jan 1 year 1.
   168  func (t *Time) sec() int64 {
   169  	if t.wall&hasMonotonic != 0 {
   170  		return wallToInternal + int64(t.wall<<1>>(nsecShift+1))
   171  	}
   172  	return t.ext
   173  }
   174  
   175  // unixSec returns the time's seconds since Jan 1 1970 (Unix time).
   176  func (t *Time) unixSec() int64 { return t.sec() + internalToUnix }
   177  
   178  // addSec adds d seconds to the time.
   179  func (t *Time) addSec(d int64) {
   180  	if t.wall&hasMonotonic != 0 {
   181  		sec := int64(t.wall << 1 >> (nsecShift + 1))
   182  		dsec := sec + d
   183  		if 0 <= dsec && dsec <= 1<<33-1 {
   184  			t.wall = t.wall&nsecMask | uint64(dsec)<<nsecShift | hasMonotonic
   185  			return
   186  		}
   187  		// Wall second now out of range for packed field.
   188  		// Move to ext.
   189  		t.stripMono()
   190  	}
   191  
   192  	// TODO: Check for overflow.
   193  	t.ext += d
   194  }
   195  
   196  // setLoc sets the location associated with the time.
   197  func (t *Time) setLoc(loc *Location) {
   198  	if loc == &utcLoc {
   199  		loc = nil
   200  	}
   201  	t.stripMono()
   202  	t.loc = loc
   203  }
   204  
   205  // stripMono strips the monotonic clock reading in t.
   206  func (t *Time) stripMono() {
   207  	if t.wall&hasMonotonic != 0 {
   208  		t.ext = t.sec()
   209  		t.wall &= nsecMask
   210  	}
   211  }
   212  
   213  // setMono sets the monotonic clock reading in t.
   214  // If t cannot hold a monotonic clock reading,
   215  // because its wall time is too large,
   216  // setMono is a no-op.
   217  func (t *Time) setMono(m int64) {
   218  	if t.wall&hasMonotonic == 0 {
   219  		sec := t.ext
   220  		if sec < minWall || maxWall < sec {
   221  			return
   222  		}
   223  		t.wall |= hasMonotonic | uint64(sec-minWall)<<nsecShift
   224  	}
   225  	t.ext = m
   226  }
   227  
   228  // mono returns t's monotonic clock reading.
   229  // It returns 0 for a missing reading.
   230  // This function is used only for testing,
   231  // so it's OK that technically 0 is a valid
   232  // monotonic clock reading as well.
   233  func (t *Time) mono() int64 {
   234  	if t.wall&hasMonotonic == 0 {
   235  		return 0
   236  	}
   237  	return t.ext
   238  }
   239  
   240  // After reports whether the time instant t is after u.
   241  func (t Time) After(u Time) bool {
   242  	if t.wall&u.wall&hasMonotonic != 0 {
   243  		return t.ext > u.ext
   244  	}
   245  	ts := t.sec()
   246  	us := u.sec()
   247  	return ts > us || ts == us && t.nsec() > u.nsec()
   248  }
   249  
   250  // Before reports whether the time instant t is before u.
   251  func (t Time) Before(u Time) bool {
   252  	if t.wall&u.wall&hasMonotonic != 0 {
   253  		return t.ext < u.ext
   254  	}
   255  	return t.sec() < u.sec() || t.sec() == u.sec() && t.nsec() < u.nsec()
   256  }
   257  
   258  // Equal reports whether t and u represent the same time instant.
   259  // Two times can be equal even if they are in different locations.
   260  // For example, 6:00 +0200 CEST and 4:00 UTC are Equal.
   261  // See the documentation on the Time type for the pitfalls of using == with
   262  // Time values; most code should use Equal instead.
   263  func (t Time) Equal(u Time) bool {
   264  	if t.wall&u.wall&hasMonotonic != 0 {
   265  		return t.ext == u.ext
   266  	}
   267  	return t.sec() == u.sec() && t.nsec() == u.nsec()
   268  }
   269  
   270  // A Month specifies a month of the year (January = 1, ...).
   271  type Month int
   272  
   273  const (
   274  	January Month = 1 + iota
   275  	February
   276  	March
   277  	April
   278  	May
   279  	June
   280  	July
   281  	August
   282  	September
   283  	October
   284  	November
   285  	December
   286  )
   287  
   288  var months = [...]string{
   289  	"January",
   290  	"February",
   291  	"March",
   292  	"April",
   293  	"May",
   294  	"June",
   295  	"July",
   296  	"August",
   297  	"September",
   298  	"October",
   299  	"November",
   300  	"December",
   301  }
   302  
   303  // String returns the English name of the month ("January", "February", ...).
   304  func (m Month) String() string {
   305  	if January <= m && m <= December {
   306  		return months[m-1]
   307  	}
   308  	buf := make([]byte, 20)
   309  	n := fmtInt(buf, uint64(m))
   310  	return "%!Month(" + string(buf[n:]) + ")"
   311  }
   312  
   313  // A Weekday specifies a day of the week (Sunday = 0, ...).
   314  type Weekday int
   315  
   316  const (
   317  	Sunday Weekday = iota
   318  	Monday
   319  	Tuesday
   320  	Wednesday
   321  	Thursday
   322  	Friday
   323  	Saturday
   324  )
   325  
   326  var days = [...]string{
   327  	"Sunday",
   328  	"Monday",
   329  	"Tuesday",
   330  	"Wednesday",
   331  	"Thursday",
   332  	"Friday",
   333  	"Saturday",
   334  }
   335  
   336  // String returns the English name of the day ("Sunday", "Monday", ...).
   337  func (d Weekday) String() string {
   338  	if Sunday <= d && d <= Saturday {
   339  		return days[d]
   340  	}
   341  	buf := make([]byte, 20)
   342  	n := fmtInt(buf, uint64(d))
   343  	return "%!Weekday(" + string(buf[n:]) + ")"
   344  }
   345  
   346  // Computations on time.
   347  //
   348  // The zero value for a Time is defined to be
   349  //	January 1, year 1, 00:00:00.000000000 UTC
   350  // which (1) looks like a zero, or as close as you can get in a date
   351  // (1-1-1 00:00:00 UTC), (2) is unlikely enough to arise in practice to
   352  // be a suitable "not set" sentinel, unlike Jan 1 1970, and (3) has a
   353  // non-negative year even in time zones west of UTC, unlike 1-1-0
   354  // 00:00:00 UTC, which would be 12-31-(-1) 19:00:00 in New York.
   355  //
   356  // The zero Time value does not force a specific epoch for the time
   357  // representation. For example, to use the Unix epoch internally, we
   358  // could define that to distinguish a zero value from Jan 1 1970, that
   359  // time would be represented by sec=-1, nsec=1e9. However, it does
   360  // suggest a representation, namely using 1-1-1 00:00:00 UTC as the
   361  // epoch, and that's what we do.
   362  //
   363  // The Add and Sub computations are oblivious to the choice of epoch.
   364  //
   365  // The presentation computations - year, month, minute, and so on - all
   366  // rely heavily on division and modulus by positive constants. For
   367  // calendrical calculations we want these divisions to round down, even
   368  // for negative values, so that the remainder is always positive, but
   369  // Go's division (like most hardware division instructions) rounds to
   370  // zero. We can still do those computations and then adjust the result
   371  // for a negative numerator, but it's annoying to write the adjustment
   372  // over and over. Instead, we can change to a different epoch so long
   373  // ago that all the times we care about will be positive, and then round
   374  // to zero and round down coincide. These presentation routines already
   375  // have to add the zone offset, so adding the translation to the
   376  // alternate epoch is cheap. For example, having a non-negative time t
   377  // means that we can write
   378  //
   379  //	sec = t % 60
   380  //
   381  // instead of
   382  //
   383  //	sec = t % 60
   384  //	if sec < 0 {
   385  //		sec += 60
   386  //	}
   387  //
   388  // everywhere.
   389  //
   390  // The calendar runs on an exact 400 year cycle: a 400-year calendar
   391  // printed for 1970-2369 will apply as well to 2370-2769. Even the days
   392  // of the week match up. It simplifies the computations to choose the
   393  // cycle boundaries so that the exceptional years are always delayed as
   394  // long as possible. That means choosing a year equal to 1 mod 400, so
   395  // that the first leap year is the 4th year, the first missed leap year
   396  // is the 100th year, and the missed missed leap year is the 400th year.
   397  // So we'd prefer instead to print a calendar for 2001-2400 and reuse it
   398  // for 2401-2800.
   399  //
   400  // Finally, it's convenient if the delta between the Unix epoch and
   401  // long-ago epoch is representable by an int64 constant.
   402  //
   403  // These three considerations—choose an epoch as early as possible, that
   404  // uses a year equal to 1 mod 400, and that is no more than 2⁶³ seconds
   405  // earlier than 1970—bring us to the year -292277022399. We refer to
   406  // this year as the absolute zero year, and to times measured as a uint64
   407  // seconds since this year as absolute times.
   408  //
   409  // Times measured as an int64 seconds since the year 1—the representation
   410  // used for Time's sec field—are called internal times.
   411  //
   412  // Times measured as an int64 seconds since the year 1970 are called Unix
   413  // times.
   414  //
   415  // It is tempting to just use the year 1 as the absolute epoch, defining
   416  // that the routines are only valid for years >= 1. However, the
   417  // routines would then be invalid when displaying the epoch in time zones
   418  // west of UTC, since it is year 0. It doesn't seem tenable to say that
   419  // printing the zero time correctly isn't supported in half the time
   420  // zones. By comparison, it's reasonable to mishandle some times in
   421  // the year -292277022399.
   422  //
   423  // All this is opaque to clients of the API and can be changed if a
   424  // better implementation presents itself.
   425  
   426  const (
   427  	// The unsigned zero year for internal calculations.
   428  	// Must be 1 mod 400, and times before it will not compute correctly,
   429  	// but otherwise can be changed at will.
   430  	absoluteZeroYear = -292277022399
   431  
   432  	// The year of the zero Time.
   433  	// Assumed by the unixToInternal computation below.
   434  	internalYear = 1
   435  
   436  	// Offsets to convert between internal and absolute or Unix times.
   437  	absoluteToInternal int64 = (absoluteZeroYear - internalYear) * 365.2425 * secondsPerDay
   438  	internalToAbsolute       = -absoluteToInternal
   439  
   440  	unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
   441  	internalToUnix int64 = -unixToInternal
   442  
   443  	wallToInternal int64 = (1884*365 + 1884/4 - 1884/100 + 1884/400) * secondsPerDay
   444  	internalToWall int64 = -wallToInternal
   445  )
   446  
   447  // IsZero reports whether t represents the zero time instant,
   448  // January 1, year 1, 00:00:00 UTC.
   449  func (t Time) IsZero() bool {
   450  	return t.sec() == 0 && t.nsec() == 0
   451  }
   452  
   453  // abs returns the time t as an absolute time, adjusted by the zone offset.
   454  // It is called when computing a presentation property like Month or Hour.
   455  func (t Time) abs() uint64 {
   456  	l := t.loc
   457  	// Avoid function calls when possible.
   458  	if l == nil || l == &localLoc {
   459  		l = l.get()
   460  	}
   461  	sec := t.unixSec()
   462  	if l != &utcLoc {
   463  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
   464  			sec += int64(l.cacheZone.offset)
   465  		} else {
   466  			_, offset, _, _ := l.lookup(sec)
   467  			sec += int64(offset)
   468  		}
   469  	}
   470  	return uint64(sec + (unixToInternal + internalToAbsolute))
   471  }
   472  
   473  // locabs is a combination of the Zone and abs methods,
   474  // extracting both return values from a single zone lookup.
   475  func (t Time) locabs() (name string, offset int, abs uint64) {
   476  	l := t.loc
   477  	if l == nil || l == &localLoc {
   478  		l = l.get()
   479  	}
   480  	// Avoid function call if we hit the local time cache.
   481  	sec := t.unixSec()
   482  	if l != &utcLoc {
   483  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
   484  			name = l.cacheZone.name
   485  			offset = l.cacheZone.offset
   486  		} else {
   487  			name, offset, _, _ = l.lookup(sec)
   488  		}
   489  		sec += int64(offset)
   490  	} else {
   491  		name = "UTC"
   492  	}
   493  	abs = uint64(sec + (unixToInternal + internalToAbsolute))
   494  	return
   495  }
   496  
   497  // Date returns the year, month, and day in which t occurs.
   498  func (t Time) Date() (year int, month Month, day int) {
   499  	year, month, day, _ = t.date(true)
   500  	return
   501  }
   502  
   503  // Year returns the year in which t occurs.
   504  func (t Time) Year() int {
   505  	year, _, _, _ := t.date(false)
   506  	return year
   507  }
   508  
   509  // Month returns the month of the year specified by t.
   510  func (t Time) Month() Month {
   511  	_, month, _, _ := t.date(true)
   512  	return month
   513  }
   514  
   515  // Day returns the day of the month specified by t.
   516  func (t Time) Day() int {
   517  	_, _, day, _ := t.date(true)
   518  	return day
   519  }
   520  
   521  // Weekday returns the day of the week specified by t.
   522  func (t Time) Weekday() Weekday {
   523  	return absWeekday(t.abs())
   524  }
   525  
   526  // absWeekday is like Weekday but operates on an absolute time.
   527  func absWeekday(abs uint64) Weekday {
   528  	// January 1 of the absolute year, like January 1 of 2001, was a Monday.
   529  	sec := (abs + uint64(Monday)*secondsPerDay) % secondsPerWeek
   530  	return Weekday(int(sec) / secondsPerDay)
   531  }
   532  
   533  // ISOWeek returns the ISO 8601 year and week number in which t occurs.
   534  // Week ranges from 1 to 53. Jan 01 to Jan 03 of year n might belong to
   535  // week 52 or 53 of year n-1, and Dec 29 to Dec 31 might belong to week 1
   536  // of year n+1.
   537  func (t Time) ISOWeek() (year, week int) {
   538  	year, month, day, yday := t.date(true)
   539  	wday := int(t.Weekday()+6) % 7 // weekday but Monday = 0.
   540  	const (
   541  		Mon int = iota
   542  		Tue
   543  		Wed
   544  		Thu
   545  		Fri
   546  		Sat
   547  		Sun
   548  	)
   549  
   550  	// Calculate week as number of Mondays in year up to
   551  	// and including today, plus 1 because the first week is week 0.
   552  	// Putting the + 1 inside the numerator as a + 7 keeps the
   553  	// numerator from being negative, which would cause it to
   554  	// round incorrectly.
   555  	week = (yday - wday + 7) / 7
   556  
   557  	// The week number is now correct under the assumption
   558  	// that the first Monday of the year is in week 1.
   559  	// If Jan 1 is a Tuesday, Wednesday, or Thursday, the first Monday
   560  	// is actually in week 2.
   561  	jan1wday := (wday - yday + 7*53) % 7
   562  	if Tue <= jan1wday && jan1wday <= Thu {
   563  		week++
   564  	}
   565  
   566  	// If the week number is still 0, we're in early January but in
   567  	// the last week of last year.
   568  	if week == 0 {
   569  		year--
   570  		week = 52
   571  		// A year has 53 weeks when Jan 1 or Dec 31 is a Thursday,
   572  		// meaning Jan 1 of the next year is a Friday
   573  		// or it was a leap year and Jan 1 of the next year is a Saturday.
   574  		if jan1wday == Fri || (jan1wday == Sat && isLeap(year)) {
   575  			week++
   576  		}
   577  	}
   578  
   579  	// December 29 to 31 are in week 1 of next year if
   580  	// they are after the last Thursday of the year and
   581  	// December 31 is a Monday, Tuesday, or Wednesday.
   582  	if month == December && day >= 29 && wday < Thu {
   583  		if dec31wday := (wday + 31 - day) % 7; Mon <= dec31wday && dec31wday <= Wed {
   584  			year++
   585  			week = 1
   586  		}
   587  	}
   588  
   589  	return
   590  }
   591  
   592  // Clock returns the hour, minute, and second within the day specified by t.
   593  func (t Time) Clock() (hour, min, sec int) {
   594  	return absClock(t.abs())
   595  }
   596  
   597  // absClock is like clock but operates on an absolute time.
   598  func absClock(abs uint64) (hour, min, sec int) {
   599  	sec = int(abs % secondsPerDay)
   600  	hour = sec / secondsPerHour
   601  	sec -= hour * secondsPerHour
   602  	min = sec / secondsPerMinute
   603  	sec -= min * secondsPerMinute
   604  	return
   605  }
   606  
   607  // Hour returns the hour within the day specified by t, in the range [0, 23].
   608  func (t Time) Hour() int {
   609  	return int(t.abs()%secondsPerDay) / secondsPerHour
   610  }
   611  
   612  // Minute returns the minute offset within the hour specified by t, in the range [0, 59].
   613  func (t Time) Minute() int {
   614  	return int(t.abs()%secondsPerHour) / secondsPerMinute
   615  }
   616  
   617  // Second returns the second offset within the minute specified by t, in the range [0, 59].
   618  func (t Time) Second() int {
   619  	return int(t.abs() % secondsPerMinute)
   620  }
   621  
   622  // Nanosecond returns the nanosecond offset within the second specified by t,
   623  // in the range [0, 999999999].
   624  func (t Time) Nanosecond() int {
   625  	return int(t.nsec())
   626  }
   627  
   628  // YearDay returns the day of the year specified by t, in the range [1,365] for non-leap years,
   629  // and [1,366] in leap years.
   630  func (t Time) YearDay() int {
   631  	_, _, _, yday := t.date(false)
   632  	return yday + 1
   633  }
   634  
   635  // A Duration represents the elapsed time between two instants
   636  // as an int64 nanosecond count. The representation limits the
   637  // largest representable duration to approximately 290 years.
   638  type Duration int64
   639  
   640  const (
   641  	minDuration Duration = -1 << 63
   642  	maxDuration Duration = 1<<63 - 1
   643  )
   644  
   645  // Common durations. There is no definition for units of Day or larger
   646  // to avoid confusion across daylight savings time zone transitions.
   647  //
   648  // To count the number of units in a Duration, divide:
   649  //	second := time.Second
   650  //	fmt.Print(int64(second/time.Millisecond)) // prints 1000
   651  //
   652  // To convert an integer number of units to a Duration, multiply:
   653  //	seconds := 10
   654  //	fmt.Print(time.Duration(seconds)*time.Second) // prints 10s
   655  //
   656  const (
   657  	Nanosecond  Duration = 1
   658  	Microsecond          = 1000 * Nanosecond
   659  	Millisecond          = 1000 * Microsecond
   660  	Second               = 1000 * Millisecond
   661  	Minute               = 60 * Second
   662  	Hour                 = 60 * Minute
   663  )
   664  
   665  // String returns a string representing the duration in the form "72h3m0.5s".
   666  // Leading zero units are omitted. As a special case, durations less than one
   667  // second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
   668  // that the leading digit is non-zero. The zero duration formats as 0s.
   669  func (d Duration) String() string {
   670  	// Largest time is 2540400h10m10.000000000s
   671  	var buf [32]byte
   672  	w := len(buf)
   673  
   674  	u := uint64(d)
   675  	neg := d < 0
   676  	if neg {
   677  		u = -u
   678  	}
   679  
   680  	if u < uint64(Second) {
   681  		// Special case: if duration is smaller than a second,
   682  		// use smaller units, like 1.2ms
   683  		var prec int
   684  		w--
   685  		buf[w] = 's'
   686  		w--
   687  		switch {
   688  		case u == 0:
   689  			return "0s"
   690  		case u < uint64(Microsecond):
   691  			// print nanoseconds
   692  			prec = 0
   693  			buf[w] = 'n'
   694  		case u < uint64(Millisecond):
   695  			// print microseconds
   696  			prec = 3
   697  			// U+00B5 'µ' micro sign == 0xC2 0xB5
   698  			w-- // Need room for two bytes.
   699  			copy(buf[w:], "µ")
   700  		default:
   701  			// print milliseconds
   702  			prec = 6
   703  			buf[w] = 'm'
   704  		}
   705  		w, u = fmtFrac(buf[:w], u, prec)
   706  		w = fmtInt(buf[:w], u)
   707  	} else {
   708  		w--
   709  		buf[w] = 's'
   710  
   711  		w, u = fmtFrac(buf[:w], u, 9)
   712  
   713  		// u is now integer seconds
   714  		w = fmtInt(buf[:w], u%60)
   715  		u /= 60
   716  
   717  		// u is now integer minutes
   718  		if u > 0 {
   719  			w--
   720  			buf[w] = 'm'
   721  			w = fmtInt(buf[:w], u%60)
   722  			u /= 60
   723  
   724  			// u is now integer hours
   725  			// Stop at hours because days can be different lengths.
   726  			if u > 0 {
   727  				w--
   728  				buf[w] = 'h'
   729  				w = fmtInt(buf[:w], u)
   730  			}
   731  		}
   732  	}
   733  
   734  	if neg {
   735  		w--
   736  		buf[w] = '-'
   737  	}
   738  
   739  	return string(buf[w:])
   740  }
   741  
   742  // fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
   743  // tail of buf, omitting trailing zeros. It omits the decimal
   744  // point too when the fraction is 0. It returns the index where the
   745  // output bytes begin and the value v/10**prec.
   746  func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
   747  	// Omit trailing zeros up to and including decimal point.
   748  	w := len(buf)
   749  	print := false
   750  	for i := 0; i < prec; i++ {
   751  		digit := v % 10
   752  		print = print || digit != 0
   753  		if print {
   754  			w--
   755  			buf[w] = byte(digit) + '0'
   756  		}
   757  		v /= 10
   758  	}
   759  	if print {
   760  		w--
   761  		buf[w] = '.'
   762  	}
   763  	return w, v
   764  }
   765  
   766  // fmtInt formats v into the tail of buf.
   767  // It returns the index where the output begins.
   768  func fmtInt(buf []byte, v uint64) int {
   769  	w := len(buf)
   770  	if v == 0 {
   771  		w--
   772  		buf[w] = '0'
   773  	} else {
   774  		for v > 0 {
   775  			w--
   776  			buf[w] = byte(v%10) + '0'
   777  			v /= 10
   778  		}
   779  	}
   780  	return w
   781  }
   782  
   783  // Nanoseconds returns the duration as an integer nanosecond count.
   784  func (d Duration) Nanoseconds() int64 { return int64(d) }
   785  
   786  // These methods return float64 because the dominant
   787  // use case is for printing a floating point number like 1.5s, and
   788  // a truncation to integer would make them not useful in those cases.
   789  // Splitting the integer and fraction ourselves guarantees that
   790  // converting the returned float64 to an integer rounds the same
   791  // way that a pure integer conversion would have, even in cases
   792  // where, say, float64(d.Nanoseconds())/1e9 would have rounded
   793  // differently.
   794  
   795  // Seconds returns the duration as a floating point number of seconds.
   796  func (d Duration) Seconds() float64 {
   797  	sec := d / Second
   798  	nsec := d % Second
   799  	return float64(sec) + float64(nsec)/1e9
   800  }
   801  
   802  // Minutes returns the duration as a floating point number of minutes.
   803  func (d Duration) Minutes() float64 {
   804  	min := d / Minute
   805  	nsec := d % Minute
   806  	return float64(min) + float64(nsec)/(60*1e9)
   807  }
   808  
   809  // Hours returns the duration as a floating point number of hours.
   810  func (d Duration) Hours() float64 {
   811  	hour := d / Hour
   812  	nsec := d % Hour
   813  	return float64(hour) + float64(nsec)/(60*60*1e9)
   814  }
   815  
   816  // Truncate returns the result of rounding d toward zero to a multiple of m.
   817  // If m <= 0, Truncate returns d unchanged.
   818  func (d Duration) Truncate(m Duration) Duration {
   819  	if m <= 0 {
   820  		return d
   821  	}
   822  	return d - d%m
   823  }
   824  
   825  // lessThanHalf reports whether x+x < y but avoids overflow,
   826  // assuming x and y are both positive (Duration is signed).
   827  func lessThanHalf(x, y Duration) bool {
   828  	return uint64(x)+uint64(x) < uint64(y)
   829  }
   830  
   831  // Round returns the result of rounding d to the nearest multiple of m.
   832  // The rounding behavior for halfway values is to round away from zero.
   833  // If the result exceeds the maximum (or minimum)
   834  // value that can be stored in a Duration,
   835  // Round returns the maximum (or minimum) duration.
   836  // If m <= 0, Round returns d unchanged.
   837  func (d Duration) Round(m Duration) Duration {
   838  	if m <= 0 {
   839  		return d
   840  	}
   841  	r := d % m
   842  	if d < 0 {
   843  		r = -r
   844  		if lessThanHalf(r, m) {
   845  			return d + r
   846  		}
   847  		if d1 := d - m + r; d1 < d {
   848  			return d1
   849  		}
   850  		return minDuration // overflow
   851  	}
   852  	if lessThanHalf(r, m) {
   853  		return d - r
   854  	}
   855  	if d1 := d + m - r; d1 > d {
   856  		return d1
   857  	}
   858  	return maxDuration // overflow
   859  }
   860  
   861  // Add returns the time t+d.
   862  func (t Time) Add(d Duration) Time {
   863  	dsec := int64(d / 1e9)
   864  	nsec := t.nsec() + int32(d%1e9)
   865  	if nsec >= 1e9 {
   866  		dsec++
   867  		nsec -= 1e9
   868  	} else if nsec < 0 {
   869  		dsec--
   870  		nsec += 1e9
   871  	}
   872  	t.wall = t.wall&^nsecMask | uint64(nsec) // update nsec
   873  	t.addSec(dsec)
   874  	if t.wall&hasMonotonic != 0 {
   875  		te := t.ext + int64(d)
   876  		if d < 0 && te > t.ext || d > 0 && te < t.ext {
   877  			// Monotonic clock reading now out of range; degrade to wall-only.
   878  			t.stripMono()
   879  		} else {
   880  			t.ext = te
   881  		}
   882  	}
   883  	return t
   884  }
   885  
   886  // Sub returns the duration t-u. If the result exceeds the maximum (or minimum)
   887  // value that can be stored in a Duration, the maximum (or minimum) duration
   888  // will be returned.
   889  // To compute t-d for a duration d, use t.Add(-d).
   890  func (t Time) Sub(u Time) Duration {
   891  	if t.wall&u.wall&hasMonotonic != 0 {
   892  		te := t.ext
   893  		ue := u.ext
   894  		d := Duration(te - ue)
   895  		if d < 0 && te > ue {
   896  			return maxDuration // t - u is positive out of range
   897  		}
   898  		if d > 0 && te < ue {
   899  			return minDuration // t - u is negative out of range
   900  		}
   901  		return d
   902  	}
   903  	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
   904  	// Check for overflow or underflow.
   905  	switch {
   906  	case u.Add(d).Equal(t):
   907  		return d // d is correct
   908  	case t.Before(u):
   909  		return minDuration // t - u is negative out of range
   910  	default:
   911  		return maxDuration // t - u is positive out of range
   912  	}
   913  }
   914  
   915  // Since returns the time elapsed since t.
   916  // It is shorthand for time.Now().Sub(t).
   917  func Since(t Time) Duration {
   918  	var now Time
   919  	if t.wall&hasMonotonic != 0 {
   920  		// Common case optimization: if t has monotomic time, then Sub will use only it.
   921  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
   922  	} else {
   923  		now = Now()
   924  	}
   925  	return now.Sub(t)
   926  }
   927  
   928  // Until returns the duration until t.
   929  // It is shorthand for t.Sub(time.Now()).
   930  func Until(t Time) Duration {
   931  	var now Time
   932  	if t.wall&hasMonotonic != 0 {
   933  		// Common case optimization: if t has monotomic time, then Sub will use only it.
   934  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
   935  	} else {
   936  		now = Now()
   937  	}
   938  	return t.Sub(now)
   939  }
   940  
   941  // AddDate returns the time corresponding to adding the
   942  // given number of years, months, and days to t.
   943  // For example, AddDate(-1, 2, 3) applied to January 1, 2011
   944  // returns March 4, 2010.
   945  //
   946  // AddDate normalizes its result in the same way that Date does,
   947  // so, for example, adding one month to October 31 yields
   948  // December 1, the normalized form for November 31.
   949  func (t Time) AddDate(years int, months int, days int) Time {
   950  	year, month, day := t.Date()
   951  	hour, min, sec := t.Clock()
   952  	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec()), t.Location())
   953  }
   954  
   955  const (
   956  	secondsPerMinute = 60
   957  	secondsPerHour   = 60 * secondsPerMinute
   958  	secondsPerDay    = 24 * secondsPerHour
   959  	secondsPerWeek   = 7 * secondsPerDay
   960  	daysPer400Years  = 365*400 + 97
   961  	daysPer100Years  = 365*100 + 24
   962  	daysPer4Years    = 365*4 + 1
   963  )
   964  
   965  // date computes the year, day of year, and when full=true,
   966  // the month and day in which t occurs.
   967  func (t Time) date(full bool) (year int, month Month, day int, yday int) {
   968  	return absDate(t.abs(), full)
   969  }
   970  
   971  // absDate is like date but operates on an absolute time.
   972  func absDate(abs uint64, full bool) (year int, month Month, day int, yday int) {
   973  	// Split into time and day.
   974  	d := abs / secondsPerDay
   975  
   976  	// Account for 400 year cycles.
   977  	n := d / daysPer400Years
   978  	y := 400 * n
   979  	d -= daysPer400Years * n
   980  
   981  	// Cut off 100-year cycles.
   982  	// The last cycle has one extra leap year, so on the last day
   983  	// of that year, day / daysPer100Years will be 4 instead of 3.
   984  	// Cut it back down to 3 by subtracting n>>2.
   985  	n = d / daysPer100Years
   986  	n -= n >> 2
   987  	y += 100 * n
   988  	d -= daysPer100Years * n
   989  
   990  	// Cut off 4-year cycles.
   991  	// The last cycle has a missing leap year, which does not
   992  	// affect the computation.
   993  	n = d / daysPer4Years
   994  	y += 4 * n
   995  	d -= daysPer4Years * n
   996  
   997  	// Cut off years within a 4-year cycle.
   998  	// The last year is a leap year, so on the last day of that year,
   999  	// day / 365 will be 4 instead of 3. Cut it back down to 3
  1000  	// by subtracting n>>2.
  1001  	n = d / 365
  1002  	n -= n >> 2
  1003  	y += n
  1004  	d -= 365 * n
  1005  
  1006  	year = int(int64(y) + absoluteZeroYear)
  1007  	yday = int(d)
  1008  
  1009  	if !full {
  1010  		return
  1011  	}
  1012  
  1013  	day = yday
  1014  	if isLeap(year) {
  1015  		// Leap year
  1016  		switch {
  1017  		case day > 31+29-1:
  1018  			// After leap day; pretend it wasn't there.
  1019  			day--
  1020  		case day == 31+29-1:
  1021  			// Leap day.
  1022  			month = February
  1023  			day = 29
  1024  			return
  1025  		}
  1026  	}
  1027  
  1028  	// Estimate month on assumption that every month has 31 days.
  1029  	// The estimate may be too low by at most one month, so adjust.
  1030  	month = Month(day / 31)
  1031  	end := int(daysBefore[month+1])
  1032  	var begin int
  1033  	if day >= end {
  1034  		month++
  1035  		begin = end
  1036  	} else {
  1037  		begin = int(daysBefore[month])
  1038  	}
  1039  
  1040  	month++ // because January is 1
  1041  	day = day - begin + 1
  1042  	return
  1043  }
  1044  
  1045  // daysBefore[m] counts the number of days in a non-leap year
  1046  // before month m begins. There is an entry for m=12, counting
  1047  // the number of days before January of next year (365).
  1048  var daysBefore = [...]int32{
  1049  	0,
  1050  	31,
  1051  	31 + 28,
  1052  	31 + 28 + 31,
  1053  	31 + 28 + 31 + 30,
  1054  	31 + 28 + 31 + 30 + 31,
  1055  	31 + 28 + 31 + 30 + 31 + 30,
  1056  	31 + 28 + 31 + 30 + 31 + 30 + 31,
  1057  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
  1058  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
  1059  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
  1060  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
  1061  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
  1062  }
  1063  
  1064  func daysIn(m Month, year int) int {
  1065  	if m == February && isLeap(year) {
  1066  		return 29
  1067  	}
  1068  	return int(daysBefore[m] - daysBefore[m-1])
  1069  }
  1070  
  1071  // Provided by package runtime.
  1072  func now() (sec int64, nsec int32, mono int64)
  1073  
  1074  // runtimeNano returns the current value of the runtime clock in nanoseconds.
  1075  //go:linkname runtimeNano runtime.nanotime
  1076  func runtimeNano() int64
  1077  
  1078  // Monotonic times are reported as offsets from startNano.
  1079  // We initialize startNano to runtimeNano() - 1 so that on systems where
  1080  // monotonic time resolution is fairly low (e.g. Windows 2008
  1081  // which appears to have a default resolution of 15ms),
  1082  // we avoid ever reporting a monotonic time of 0.
  1083  // (Callers may want to use 0 as "time not set".)
  1084  var startNano int64 = runtimeNano() - 1
  1085  
  1086  // Now returns the current local time.
  1087  func Now() Time {
  1088  	sec, nsec, mono := now()
  1089  	mono -= startNano
  1090  	sec += unixToInternal - minWall
  1091  	if uint64(sec)>>33 != 0 {
  1092  		return Time{uint64(nsec), sec + minWall, Local}
  1093  	}
  1094  	return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}
  1095  }
  1096  
  1097  func unixTime(sec int64, nsec int32) Time {
  1098  	return Time{uint64(nsec), sec + unixToInternal, Local}
  1099  }
  1100  
  1101  // UTC returns t with the location set to UTC.
  1102  func (t Time) UTC() Time {
  1103  	t.setLoc(&utcLoc)
  1104  	return t
  1105  }
  1106  
  1107  // Local returns t with the location set to local time.
  1108  func (t Time) Local() Time {
  1109  	t.setLoc(Local)
  1110  	return t
  1111  }
  1112  
  1113  // In returns a copy of t representing the same time instant, but
  1114  // with the copy's location information set to loc for display
  1115  // purposes.
  1116  //
  1117  // In panics if loc is nil.
  1118  func (t Time) In(loc *Location) Time {
  1119  	if loc == nil {
  1120  		panic("time: missing Location in call to Time.In")
  1121  	}
  1122  	t.setLoc(loc)
  1123  	return t
  1124  }
  1125  
  1126  // Location returns the time zone information associated with t.
  1127  func (t Time) Location() *Location {
  1128  	l := t.loc
  1129  	if l == nil {
  1130  		l = UTC
  1131  	}
  1132  	return l
  1133  }
  1134  
  1135  // Zone computes the time zone in effect at time t, returning the abbreviated
  1136  // name of the zone (such as "CET") and its offset in seconds east of UTC.
  1137  func (t Time) Zone() (name string, offset int) {
  1138  	name, offset, _, _ = t.loc.lookup(t.unixSec())
  1139  	return
  1140  }
  1141  
  1142  // Unix returns t as a Unix time, the number of seconds elapsed
  1143  // since January 1, 1970 UTC. The result does not depend on the
  1144  // location associated with t.
  1145  func (t Time) Unix() int64 {
  1146  	return t.unixSec()
  1147  }
  1148  
  1149  // UnixNano returns t as a Unix time, the number of nanoseconds elapsed
  1150  // since January 1, 1970 UTC. The result is undefined if the Unix time
  1151  // in nanoseconds cannot be represented by an int64 (a date before the year
  1152  // 1678 or after 2262). Note that this means the result of calling UnixNano
  1153  // on the zero Time is undefined. The result does not depend on the
  1154  // location associated with t.
  1155  func (t Time) UnixNano() int64 {
  1156  	return (t.unixSec())*1e9 + int64(t.nsec())
  1157  }
  1158  
  1159  const timeBinaryVersion byte = 1
  1160  
  1161  // MarshalBinary implements the encoding.BinaryMarshaler interface.
  1162  func (t Time) MarshalBinary() ([]byte, error) {
  1163  	var offsetMin int16 // minutes east of UTC. -1 is UTC.
  1164  
  1165  	if t.Location() == UTC {
  1166  		offsetMin = -1
  1167  	} else {
  1168  		_, offset := t.Zone()
  1169  		if offset%60 != 0 {
  1170  			return nil, errors.New("Time.MarshalBinary: zone offset has fractional minute")
  1171  		}
  1172  		offset /= 60
  1173  		if offset < -32768 || offset == -1 || offset > 32767 {
  1174  			return nil, errors.New("Time.MarshalBinary: unexpected zone offset")
  1175  		}
  1176  		offsetMin = int16(offset)
  1177  	}
  1178  
  1179  	sec := t.sec()
  1180  	nsec := t.nsec()
  1181  	enc := []byte{
  1182  		timeBinaryVersion, // byte 0 : version
  1183  		byte(sec >> 56),   // bytes 1-8: seconds
  1184  		byte(sec >> 48),
  1185  		byte(sec >> 40),
  1186  		byte(sec >> 32),
  1187  		byte(sec >> 24),
  1188  		byte(sec >> 16),
  1189  		byte(sec >> 8),
  1190  		byte(sec),
  1191  		byte(nsec >> 24), // bytes 9-12: nanoseconds
  1192  		byte(nsec >> 16),
  1193  		byte(nsec >> 8),
  1194  		byte(nsec),
  1195  		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
  1196  		byte(offsetMin),
  1197  	}
  1198  
  1199  	return enc, nil
  1200  }
  1201  
  1202  // UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
  1203  func (t *Time) UnmarshalBinary(data []byte) error {
  1204  	buf := data
  1205  	if len(buf) == 0 {
  1206  		return errors.New("Time.UnmarshalBinary: no data")
  1207  	}
  1208  
  1209  	if buf[0] != timeBinaryVersion {
  1210  		return errors.New("Time.UnmarshalBinary: unsupported version")
  1211  	}
  1212  
  1213  	if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
  1214  		return errors.New("Time.UnmarshalBinary: invalid length")
  1215  	}
  1216  
  1217  	buf = buf[1:]
  1218  	sec := int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
  1219  		int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56
  1220  
  1221  	buf = buf[8:]
  1222  	nsec := int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
  1223  
  1224  	buf = buf[4:]
  1225  	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
  1226  
  1227  	*t = Time{}
  1228  	t.wall = uint64(nsec)
  1229  	t.ext = sec
  1230  
  1231  	if offset == -1*60 {
  1232  		t.setLoc(&utcLoc)
  1233  	} else if _, localoff, _, _ := Local.lookup(t.unixSec()); offset == localoff {
  1234  		t.setLoc(Local)
  1235  	} else {
  1236  		t.setLoc(FixedZone("", offset))
  1237  	}
  1238  
  1239  	return nil
  1240  }
  1241  
  1242  // TODO(rsc): Remove GobEncoder, GobDecoder, MarshalJSON, UnmarshalJSON in Go 2.
  1243  // The same semantics will be provided by the generic MarshalBinary, MarshalText,
  1244  // UnmarshalBinary, UnmarshalText.
  1245  
  1246  // GobEncode implements the gob.GobEncoder interface.
  1247  func (t Time) GobEncode() ([]byte, error) {
  1248  	return t.MarshalBinary()
  1249  }
  1250  
  1251  // GobDecode implements the gob.GobDecoder interface.
  1252  func (t *Time) GobDecode(data []byte) error {
  1253  	return t.UnmarshalBinary(data)
  1254  }
  1255  
  1256  // MarshalJSON implements the json.Marshaler interface.
  1257  // The time is a quoted string in RFC 3339 format, with sub-second precision added if present.
  1258  func (t Time) MarshalJSON() ([]byte, error) {
  1259  	if y := t.Year(); y < 0 || y >= 10000 {
  1260  		// RFC 3339 is clear that years are 4 digits exactly.
  1261  		// See golang.org/issue/4556#c15 for more discussion.
  1262  		return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
  1263  	}
  1264  
  1265  	b := make([]byte, 0, len(RFC3339Nano)+2)
  1266  	b = append(b, '"')
  1267  	b = t.AppendFormat(b, RFC3339Nano)
  1268  	b = append(b, '"')
  1269  	return b, nil
  1270  }
  1271  
  1272  // UnmarshalJSON implements the json.Unmarshaler interface.
  1273  // The time is expected to be a quoted string in RFC 3339 format.
  1274  func (t *Time) UnmarshalJSON(data []byte) error {
  1275  	// Ignore null, like in the main JSON package.
  1276  	if string(data) == "null" {
  1277  		return nil
  1278  	}
  1279  	// Fractional seconds are handled implicitly by Parse.
  1280  	var err error
  1281  	*t, err = Parse(`"`+RFC3339+`"`, string(data))
  1282  	return err
  1283  }
  1284  
  1285  // MarshalText implements the encoding.TextMarshaler interface.
  1286  // The time is formatted in RFC 3339 format, with sub-second precision added if present.
  1287  func (t Time) MarshalText() ([]byte, error) {
  1288  	if y := t.Year(); y < 0 || y >= 10000 {
  1289  		return nil, errors.New("Time.MarshalText: year outside of range [0,9999]")
  1290  	}
  1291  
  1292  	b := make([]byte, 0, len(RFC3339Nano))
  1293  	return t.AppendFormat(b, RFC3339Nano), nil
  1294  }
  1295  
  1296  // UnmarshalText implements the encoding.TextUnmarshaler interface.
  1297  // The time is expected to be in RFC 3339 format.
  1298  func (t *Time) UnmarshalText(data []byte) error {
  1299  	// Fractional seconds are handled implicitly by Parse.
  1300  	var err error
  1301  	*t, err = Parse(RFC3339, string(data))
  1302  	return err
  1303  }
  1304  
  1305  // Unix returns the local Time corresponding to the given Unix time,
  1306  // sec seconds and nsec nanoseconds since January 1, 1970 UTC.
  1307  // It is valid to pass nsec outside the range [0, 999999999].
  1308  // Not all sec values have a corresponding time value. One such
  1309  // value is 1<<63-1 (the largest int64 value).
  1310  func Unix(sec int64, nsec int64) Time {
  1311  	if nsec < 0 || nsec >= 1e9 {
  1312  		n := nsec / 1e9
  1313  		sec += n
  1314  		nsec -= n * 1e9
  1315  		if nsec < 0 {
  1316  			nsec += 1e9
  1317  			sec--
  1318  		}
  1319  	}
  1320  	return unixTime(sec, int32(nsec))
  1321  }
  1322  
  1323  func isLeap(year int) bool {
  1324  	return year%4 == 0 && (year%100 != 0 || year%400 == 0)
  1325  }
  1326  
  1327  // norm returns nhi, nlo such that
  1328  //	hi * base + lo == nhi * base + nlo
  1329  //	0 <= nlo < base
  1330  func norm(hi, lo, base int) (nhi, nlo int) {
  1331  	if lo < 0 {
  1332  		n := (-lo-1)/base + 1
  1333  		hi -= n
  1334  		lo += n * base
  1335  	}
  1336  	if lo >= base {
  1337  		n := lo / base
  1338  		hi += n
  1339  		lo -= n * base
  1340  	}
  1341  	return hi, lo
  1342  }
  1343  
  1344  // Date returns the Time corresponding to
  1345  //	yyyy-mm-dd hh:mm:ss + nsec nanoseconds
  1346  // in the appropriate zone for that time in the given location.
  1347  //
  1348  // The month, day, hour, min, sec, and nsec values may be outside
  1349  // their usual ranges and will be normalized during the conversion.
  1350  // For example, October 32 converts to November 1.
  1351  //
  1352  // A daylight savings time transition skips or repeats times.
  1353  // For example, in the United States, March 13, 2011 2:15am never occurred,
  1354  // while November 6, 2011 1:15am occurred twice. In such cases, the
  1355  // choice of time zone, and therefore the time, is not well-defined.
  1356  // Date returns a time that is correct in one of the two zones involved
  1357  // in the transition, but it does not guarantee which.
  1358  //
  1359  // Date panics if loc is nil.
  1360  func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
  1361  	if loc == nil {
  1362  		panic("time: missing Location in call to Date")
  1363  	}
  1364  
  1365  	// Normalize month, overflowing into year.
  1366  	m := int(month) - 1
  1367  	year, m = norm(year, m, 12)
  1368  	month = Month(m) + 1
  1369  
  1370  	// Normalize nsec, sec, min, hour, overflowing into day.
  1371  	sec, nsec = norm(sec, nsec, 1e9)
  1372  	min, sec = norm(min, sec, 60)
  1373  	hour, min = norm(hour, min, 60)
  1374  	day, hour = norm(day, hour, 24)
  1375  
  1376  	y := uint64(int64(year) - absoluteZeroYear)
  1377  
  1378  	// Compute days since the absolute epoch.
  1379  
  1380  	// Add in days from 400-year cycles.
  1381  	n := y / 400
  1382  	y -= 400 * n
  1383  	d := daysPer400Years * n
  1384  
  1385  	// Add in 100-year cycles.
  1386  	n = y / 100
  1387  	y -= 100 * n
  1388  	d += daysPer100Years * n
  1389  
  1390  	// Add in 4-year cycles.
  1391  	n = y / 4
  1392  	y -= 4 * n
  1393  	d += daysPer4Years * n
  1394  
  1395  	// Add in non-leap years.
  1396  	n = y
  1397  	d += 365 * n
  1398  
  1399  	// Add in days before this month.
  1400  	d += uint64(daysBefore[month-1])
  1401  	if isLeap(year) && month >= March {
  1402  		d++ // February 29
  1403  	}
  1404  
  1405  	// Add in days before today.
  1406  	d += uint64(day - 1)
  1407  
  1408  	// Add in time elapsed today.
  1409  	abs := d * secondsPerDay
  1410  	abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)
  1411  
  1412  	unix := int64(abs) + (absoluteToInternal + internalToUnix)
  1413  
  1414  	// Look for zone offset for t, so we can adjust to UTC.
  1415  	// The lookup function expects UTC, so we pass t in the
  1416  	// hope that it will not be too close to a zone transition,
  1417  	// and then adjust if it is.
  1418  	_, offset, start, end := loc.lookup(unix)
  1419  	if offset != 0 {
  1420  		switch utc := unix - int64(offset); {
  1421  		case utc < start:
  1422  			_, offset, _, _ = loc.lookup(start - 1)
  1423  		case utc >= end:
  1424  			_, offset, _, _ = loc.lookup(end)
  1425  		}
  1426  		unix -= int64(offset)
  1427  	}
  1428  
  1429  	t := unixTime(unix, int32(nsec))
  1430  	t.setLoc(loc)
  1431  	return t
  1432  }
  1433  
  1434  // Truncate returns the result of rounding t down to a multiple of d (since the zero time).
  1435  // If d <= 0, Truncate returns t stripped of any monotonic clock reading but otherwise unchanged.
  1436  //
  1437  // Truncate operates on the time as an absolute duration since the
  1438  // zero time; it does not operate on the presentation form of the
  1439  // time. Thus, Truncate(Hour) may return a time with a non-zero
  1440  // minute, depending on the time's Location.
  1441  func (t Time) Truncate(d Duration) Time {
  1442  	t.stripMono()
  1443  	if d <= 0 {
  1444  		return t
  1445  	}
  1446  	_, r := div(t, d)
  1447  	return t.Add(-r)
  1448  }
  1449  
  1450  // Round returns the result of rounding t to the nearest multiple of d (since the zero time).
  1451  // The rounding behavior for halfway values is to round up.
  1452  // If d <= 0, Round returns t stripped of any monotonic clock reading but otherwise unchanged.
  1453  //
  1454  // Round operates on the time as an absolute duration since the
  1455  // zero time; it does not operate on the presentation form of the
  1456  // time. Thus, Round(Hour) may return a time with a non-zero
  1457  // minute, depending on the time's Location.
  1458  func (t Time) Round(d Duration) Time {
  1459  	t.stripMono()
  1460  	if d <= 0 {
  1461  		return t
  1462  	}
  1463  	_, r := div(t, d)
  1464  	if lessThanHalf(r, d) {
  1465  		return t.Add(-r)
  1466  	}
  1467  	return t.Add(d - r)
  1468  }
  1469  
  1470  // div divides t by d and returns the quotient parity and remainder.
  1471  // We don't use the quotient parity anymore (round half up instead of round to even)
  1472  // but it's still here in case we change our minds.
  1473  func div(t Time, d Duration) (qmod2 int, r Duration) {
  1474  	neg := false
  1475  	nsec := t.nsec()
  1476  	sec := t.sec()
  1477  	if sec < 0 {
  1478  		// Operate on absolute value.
  1479  		neg = true
  1480  		sec = -sec
  1481  		nsec = -nsec
  1482  		if nsec < 0 {
  1483  			nsec += 1e9
  1484  			sec-- // sec >= 1 before the -- so safe
  1485  		}
  1486  	}
  1487  
  1488  	switch {
  1489  	// Special case: 2d divides 1 second.
  1490  	case d < Second && Second%(d+d) == 0:
  1491  		qmod2 = int(nsec/int32(d)) & 1
  1492  		r = Duration(nsec % int32(d))
  1493  
  1494  	// Special case: d is a multiple of 1 second.
  1495  	case d%Second == 0:
  1496  		d1 := int64(d / Second)
  1497  		qmod2 = int(sec/d1) & 1
  1498  		r = Duration(sec%d1)*Second + Duration(nsec)
  1499  
  1500  	// General case.
  1501  	// This could be faster if more cleverness were applied,
  1502  	// but it's really only here to avoid special case restrictions in the API.
  1503  	// No one will care about these cases.
  1504  	default:
  1505  		// Compute nanoseconds as 128-bit number.
  1506  		sec := uint64(sec)
  1507  		tmp := (sec >> 32) * 1e9
  1508  		u1 := tmp >> 32
  1509  		u0 := tmp << 32
  1510  		tmp = (sec & 0xFFFFFFFF) * 1e9
  1511  		u0x, u0 := u0, u0+tmp
  1512  		if u0 < u0x {
  1513  			u1++
  1514  		}
  1515  		u0x, u0 = u0, u0+uint64(nsec)
  1516  		if u0 < u0x {
  1517  			u1++
  1518  		}
  1519  
  1520  		// Compute remainder by subtracting r<<k for decreasing k.
  1521  		// Quotient parity is whether we subtract on last round.
  1522  		d1 := uint64(d)
  1523  		for d1>>63 != 1 {
  1524  			d1 <<= 1
  1525  		}
  1526  		d0 := uint64(0)
  1527  		for {
  1528  			qmod2 = 0
  1529  			if u1 > d1 || u1 == d1 && u0 >= d0 {
  1530  				// subtract
  1531  				qmod2 = 1
  1532  				u0x, u0 = u0, u0-d0
  1533  				if u0 > u0x {
  1534  					u1--
  1535  				}
  1536  				u1 -= d1
  1537  			}
  1538  			if d1 == 0 && d0 == uint64(d) {
  1539  				break
  1540  			}
  1541  			d0 >>= 1
  1542  			d0 |= (d1 & 1) << 63
  1543  			d1 >>= 1
  1544  		}
  1545  		r = Duration(u0)
  1546  	}
  1547  
  1548  	if neg && r != 0 {
  1549  		// If input was negative and not an exact multiple of d, we computed q, r such that
  1550  		//	q*d + r = -t
  1551  		// But the right answers are given by -(q-1), d-r:
  1552  		//	q*d + r = -t
  1553  		//	-q*d - r = t
  1554  		//	-(q-1)*d + (d - r) = t
  1555  		qmod2 ^= 1
  1556  		r = d - r
  1557  	}
  1558  	return
  1559  }
  1560  

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