time: change values set at year-month layout

[tredoe]

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

$ go version
go version go1.11.5 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

Set a year-month layout with a day to zero.
https://play.golang.org/p/NYVZhLzzicL

What did you expect to see?

When I set a time to time.Date(2010, time.January, 0, 0, 0, 0, 0, time.UTC), I expected to see 2010-01-01 00:00:00 +0000 UTC

What did you see instead?

2009-12-31 00:00:00 +0000 UTC

So, to get a valid date, it changes both year and month -which had been set by me-, instead of changing the day value to 1.

[antong]

The manual says:

The month, day, hour, min, sec, and nsec values may be outside their usual ranges and will be normalized during the conversion. For example, October 32 converts to November 1.

So, your result is expected. A 0 day doesn't mean "not set", but one day less than the first day.

[tredoe]

Although the documentation says it, it's inconsistent that a date set to "2010-1-0" been modified to "2009-12-31", when the most logical is that it been changed to "2010-1-1".

[andybons]

Thanks for the report.

I understand that it can be confusing, but changing the behavior would break the Go 1 compatibility promise for the API. There's really nothing we can do. Sorry.

[tredoe]

Added a proposal for Go 2: #30086