Git bisect points to 8776be1

That change (https://golang.org/cl/78735) changes the string _1 to mean the month with space padding. Since your format string 20060102_150405 contains _1, the generated result has changed. Of course you meant it to see 15 rather than _1.

Since the use of _1 is a bit obscure, my inclination is to roll back the change rather than break (admittedly obscure) existing code. But we could also document this clearly in the release notes.

Any other opinions? CC @robpike @hallazzang

I know breaking someone's existing code is not good, I just wanted to make consistent API as I said in #22802. Since the result of this code(https://play.golang.org/p/3fAVlR36KIf) also seems not proper for you(on go 1.9.2), I can't find the best solution for this problem.

Here are what I can imagine as the second best solutions:

• Introduce a special escape character like \ as in fmt.Printf
  • Yes, it sounds ugly
• Use other characters for spacing character, like ^ that can be considered as rarely used in time layout, instead of current _.
• Leave the time.Format as it was before that change, and add new method to format(in a way like strftime does)
  • I always thought it'd be good for Go to have a strftime method. This could make both happy for those who have written old(I mean, before that change) code and those who will write new(strftime-like) code.

Except for the last suggestion, those two above should be in Go 2 milestone since there should be a guarantee that it'll not break the existing code in Go 1.

And you can give a look at this code, which might be DEFINITELY not the one you were looking for: https://play.golang.org/p/3G7dNIPYdMo

I think it might be best to roll back for 1.10 and approach with a longer lead-up in the next release, if at all.

Ok, that looks fine too.

Since the use of _1 is a bit obscure, my inclination is to roll back the change rather than break (admittedly obscure) existing code

It's not that obscure. This bit me today and had me debugging for quite some time till I realised this was a bug in master. I believe something like time.Now().UTC().Format("2006-01-02_15-04-05") can be pretty common.

Failing code - https://github.com/agnivade/funnel/blob/master/rollup.go#L19
Travis failure here - https://travis-ci.org/agnivade/funnel/jobs/324477279

tt := time.Now().UTC()
t.Log(tt.Format("2006-01-02_15-04-05"))
t.Log(tt.Format("2006-01-02#15-04-05"))

gives

rollup_test.go:48: 2018-01-03 127-56-27
rollup_test.go:49: 2018-01-03#07-56-27

The results are pretty drastically different. "_07" to " 127". I would recommend rollback.

Change https://golang.org/cl/85998 mentions this issue: time: revert CL 78735 (was: space padding using underscore)

Thanks for reverting, for the record I was also affected by this (format string : 20060102_150405).