// IsDST returns a boolean flag for a given Time that indicates if the Time is in a DST zone for the Location
func(l *Location) IsDST(t Time) bool {
	sec := t.Unix()
	_, _, _, _, isDST := l.lookup(sec)
	return isDST
}

// lookup returns information about the time zone in use at an
// instant in time expressed as seconds since January 1, 1970 00:00:00 UTC.
//
// The returned information gives the name of the zone (such as "CET"),
// the start and end times bracketing sec when that zone is in effect,
// the offset in seconds east of UTC (such as -5*60*60), and whether
// the daylight savings is being observed at that time.
func (l *Location) lookup(sec int64) (name string, offset int, start, end int64, isDST bool) {
	l = l.get()

	if len(l.zone) == 0 {
		name = "UTC"
		offset = 0
		start = alpha
		end = omega
		isDST = false
		return
	}

	if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
		name = zone.name
		offset = zone.offset
		start = l.cacheStart
		end = l.cacheEnd
		isDST = zone.isDST
		return
	}

	if len(l.tx) == 0 || sec < l.tx[0].when {
		zone := &l.zone[l.lookupFirstZone()]
		name = zone.name
		offset = zone.offset
		start = alpha
		if len(l.tx) > 0 {
			end = l.tx[0].when
		} else {
			end = omega
		}
		isDST = zone.isDST
		return
	}

	// Binary search for entry with largest time <= sec.
	// Not using sort.Search to avoid dependencies.
	tx := l.tx
	end = omega
	lo := 0
	hi := len(tx)
	for hi-lo > 1 {
		m := lo + (hi-lo)/2
		lim := tx[m].when
		if sec < lim {
			end = lim
			hi = m
		} else {
			lo = m
		}
	}

	zone := &l.zone[tx[lo].index]
	name = zone.name
	offset = zone.offset
	start = tx[lo].when
	// end = maintained during the search
	isDST = zone.isDST

	// If we're at the end of the known zone transitions,
	// try the extend string.
	if lo == len(tx)-1 && l.extend != "" {
		if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, end, sec); ok {
			return ename, eoffset, estart, eendm eisDST
		}
	}

	return
}
...
// tzset takes a timezone string like the one found in the TZ environment
// variable, the end of the last time zone transition expressed as seconds
// since January 1, 1970 00:00:00 UTC, and a time expressed the same way.
// We call this a tzset string since in C the function tzset reads TZ.
// The return values are as for lookup, plus ok which reports whether the
// parse succeeded.
func tzset(s string, initEnd, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
	var (
		stdName, dstName     string
		stdOffset, dstOffset int
	)

	stdName, s, ok = tzsetName(s)
	if ok {
		stdOffset, s, ok = tzsetOffset(s)
	}
	if !ok {
		return "", 0, 0, 0, false, false
	}

	// The numbers in the tzset string are added to local time to get UTC,
	// but our offsets are added to UTC to get local time,
	// so we negate the number we see here.
	stdOffset = -stdOffset

	if len(s) == 0 || s[0] == ',' {
		// No daylight savings time.
		return stdName, stdOffset, initEnd, omega, false, true
	}

	dstName, s, ok = tzsetName(s)
	if ok {
		if len(s) == 0 || s[0] == ',' {
			dstOffset = stdOffset + secondsPerHour
		} else {
			dstOffset, s, ok = tzsetOffset(s)
			dstOffset = -dstOffset // as with stdOffset, above
		}
	}
	if !ok {
		return "", 0, 0, 0, false, false
	}

	if len(s) == 0 {
		// Default DST rules per tzcode.
		s = ",M3.2.0,M11.1.0"
	}
	// The TZ definition does not mention ';' here but tzcode accepts it.
	if s[0] != ',' && s[0] != ';' {
		return "", 0, 0, 0, false, false
	}
	s = s[1:]

	var startRule, endRule rule
	startRule, s, ok = tzsetRule(s)
	if !ok || len(s) == 0 || s[0] != ',' {
		return "", 0, 0, 0, false, false
	}
	s = s[1:]
	endRule, s, ok = tzsetRule(s)
	if !ok || len(s) > 0 {
		return "", 0, 0, 0, false, false
	}

	year, _, _, yday := absDate(uint64(sec+unixToInternal+internalToAbsolute), false)

	ysec := int64(yday*secondsPerDay) + sec%secondsPerDay

	// Compute start of year in seconds since Unix epoch.
	d := daysSinceEpoch(year)
	abs := int64(d * secondsPerDay)
	abs += absoluteToInternal + internalToUnix

	startSec := int64(tzruleTime(year, startRule, stdOffset))
	endSec := int64(tzruleTime(year, endRule, dstOffset))
	if endSec < startSec {
		startSec, endSec = endSec, startSec
		stdName, dstName = dstName, stdName
		stdOffset, dstOffset = dstOffset, stdOffset
	}

	// The start and end values that we return are accurate
	// close to a daylight savings transition, but are otherwise
	// just the start and end of the year. That suffices for
	// the only caller that cares, which is Date.
	if ysec < startSec {
		return stdName, stdOffset, abs, startSec + abs, false, true
	} else if ysec >= endSec {
		return stdName, stdOffset, endSec + abs, abs + 365*secondsPerDay, false, true
	} else {
		return dstName, dstOffset, startSec + abs, endSec + abs, true, true
	}
}