time: Time.Format() should fail when the layout string is unexpected

[jqll]

What version of Go are you using (go version)?

$ go version
go1.13

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOHOSTOS="linux"

What did you do?

On 2019-10-18, run time.Now().Format("2006-01-01") (instead of the correct magic string "2006-01-02")

What did you expect to see?

1. Go magically understands the format and returns the correct time with this format.
   or
2. It complains the given format string is unexpected, either during compiling or at run time.

What did you see instead?

It returns a wrong time "2019-10-10".

[xhit]

Is not "magical".

When you write time.Now().Format("2006-01-01") you are printing the month twice, so the format and output are correct.

[bradfitz]

Sorry, but this is working as designed & documented.

We're not going to add magic (it's not clear what magic would work), and we can't make it return an error on duplicate layout rules, as that would likely break other people's (correct, if odd) programs.

Maybe there could be a vet or lint check for bizarre and likely incorrect format strings, though.

I wouldn't be surprised if @dominikh has already written such a thing.

[dominikh]

Actually I haven't. I would have the same issue on my end, trying to discern weird formats from incorrect formats.

Maybe one day editors will become powerful enough to show example formatted times in tooltips.

[jqll]

Thanks all! In the meantime, it's probably worth to explicitly warn in the doc https://golang.org/pkg/time/#Time.Format that the reference time must be "Mon Jan 2 15:04:05 -0700 MST 2006". Using a different time can lead to undefined behavior, not necessarily an error. Further explaining "01" = month, "02" = date will make things much more clear.

[bradfitz]

It already says that at the URL you just linked to.

[jqll]

I wish the consequence of using an arbitrary date is explicitly documented. When I read the doc, I thought using an arbitrary date will either work or give an error. So I tried and thought it worked, as there was no error. The fact that "Mon Jan 2 15:04:05 -0700 MST 2006" itself looks arbitrary and is not explained also encourages the attempt.

I'm probably biased by my experience (which is largely due to my fault), but unintentionally returning wrong timestamp can be dangerous. I wish people are warned about it even if they just skim through the doc.

[ianlancetaylor]

Do you have a specific suggestion for how the docs should be improved? It seems hard to fix the docs to work for people who just skim them. Thanks.

[jqll]

It would be very helpful if the reference time is explained and the fact that "01" = month and "02"= date is explicitly mentioned.

People can make different guesses of the underlying mechanism by reading "Format returns a textual representation of the time value formatted according to layout, which defines the format by showing how the reference time, defined to be Mon Jan 2 15:04:05 -0700 MST 2006 would be displayed if it were the value"

A very reasonable understanding is that the time should be "Mon Jan 2 15:04:05 -0700 MST 2006", so "2006-01-02 01:04:05 AMT" when expressing in AMT. But we get "2019-10-20 10:04:05 AMT" with the following code:

t, err := time.Parse("2006-01-02 15:04:05 MST", "2019-10-20 20:04:05 AMT")
if err != nil { return err }
fmt.Printf("%s", t.Format("2006-01-02 01:04:05 AMT"))

It's obvious once we know "01" = month and "02" = date. But otherwise it's surprising. I'm still worried that I can make this mistake without being warned. But making the doc very clear can help.

[tv42]

@dominikh I'd think a rough heuristic for time formats to be that they shouldn't contain repeats of the same segment (01 etc). That'd be technically more lint than vet, unfortunately, because of joke code like

rebeccaBlack := time.Now().Format("It's Monday, Monday\nGotta get down on Monday")

But I really can't think of a non-joke example. Might be worth running through the corpus and just enabling it as a vet if nothing matches -- jokes can always add overrides or ignore vet results.

[gopherbot]

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)