time AddDate(0, -1, 0) does not work for March.

[happilymarrieddad]

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

$ go version
go version go1.12.1 linux/amd64

Does this issue reproduce with the latest release?

Yes, I tested it on golang.org and my local machine

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/nick/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/nick/Projects/Go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build101166364=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I'm trying to go back 1 month from March and I get a really weird result. It goes from March 30th to March 2nd. I'm assuming it's taking the number of days in February and subtracting that from March. It should take the number of days in March and subtract it.

https://play.golang.org/p/n2xvzVFnA4G

What did you expect to see?

It should have been a date in February at least. Momentjs, not I don't like javascript, sets it to February 28 which is a little weird but at least it's February.

What did you see instead?

It set it to March 2nd.

[happilymarrieddad]

I'm going to try to tackle this one. I think I might be able to fix it. I'm pretty sure I know what the problem is.

I think the result is complying with the documentation:

AddDate normalizes its result in the same way that Date does, so, for example, adding one month to October 31 yields December 1, the normalized form for November 31.

So in your example: Mar-30-2019 - 01-00-0000 = Feb-30-2019, which, normalized, is Mar-02-2019.

[happilymarrieddad]

Oh ok, so it's supposed to do that. Is there a way to "unnormalize" it? Yea, I just got all my contrib all set up and was looking at this part of the code when I saw this message.

[happilymarrieddad]

It would seem to me that the proper solution would be the number of days in a month vs the day set after the month has been decremented and whatever is smaller is the day you pick. I'm not a date expert though. Anyways, thanks!

[ianlancetaylor]

Closing because this is working as documented and expected.

[vysdel]

Closing because this is working as documented and expected.

But how this behavior is expected?
Please check this https://play.golang.org/p/99iEgshLy6T

Sub one month from 2019 March 30 results in March 02
Sub one month from 2019 April 01 results in March 01

This is definitely a bug. It must be Feb 28. duckduckgo.com as well as other various resources show Feb 28

[happilymarrieddad]

@vysdel Yea, I agree but the masters that be seem to not think so. I was going to write my own date library to handle it the way I think is proper. Probably not the best solution but I need it to handle dates another way. I'll post here I guess when I finish my package.

[ianlancetaylor]

@vysdel Subtracting one month from March 30 gives you February 30. The date February 30 does not exist, so it is normalized, as documented for time.Time.AddDate and time.Date, to March 2.

Subtracting one month from April 1 gives you March 1. That date does exist, so no normalization is required.

What are you really trying to do?

[happilymarrieddad]

Please note: before you read this message, that Moment.js and Java do not exhibit this behavior.

@iamoryanmoshe I think what he and I am saying is that you say it's documented but there is no explanation. The term normalization means means changing something into a standard. I've never heard of the standard you refer to where you go back one month and you get a date in the same month... Also if you go forward from January you end up in March. How is going from January to March in any sense of the word going forward 1 month... I've done quite a bit of google searching and I can't find any examples of this behavior. It looks like to me someone didn't want to implement the extremely complicated cases. I've been working on writing a package for it and I've already spent a lot of time on it. There are a lot of things to consider like leap year, yes this affects it because of Feb 29, and all the other cases. You ask "What are you really trying to do?" Did you look at the two examples that are provided? I hope this helps explain the situation and why we are frustrated with the responses that we've been given. Thanks so much for taking the time to read this.

[happilymarrieddad]

[agnivade]

@happilymarrieddad - Thank you for voicing your concerns. I can assure you that a lot of thought has went into designing the time package and it's not just a matter of not wanting to "implement the extremely complicated cases."

The term "normalization" is used in various contexts. In this context, it just means dates which overflow or underflow are just adjusted to their future or past dates accordingly. If there is a more apt term for this, please feel free to send a PR so that we can update our docs.

There are various scenarios where such a behavior is helpful. For eg, I want to figure out the last day of any month. Normally, it won't be very straightforward because some of them are 30, some are 31, and then there are leap years, DST adjustments and so on. But if dates are normalized, then I can just add 1 month to the current date, and set the day to 0. This effectively goes back 1 day from the first day of the next month, giving you the last day of the current month.

If d is your current date, then
monthEnd := time.Date(d.Year(), d.Month() + 1, 0, 0, 0, 0, 0, d.Location())

In effect, the Go time package treats dates like a number line, where adding/subtracting an amount flows over to the next month, rather than wrapping around or throwing an error.

[ianlancetaylor]

@happilymarrieddad I see the examples but I don't know what you are trying to do. What does "I'm trying to go back 1 month from March" mean exactly? What precisely do you mean by "back 1 month from March 30"? To me that does not have a precise meaning. But since we wanted to provide an AddDate method, we had to pick a meaning: go back to February 30, and then figure out what date that is. You want something else, but what is it that you want?

[happilymarrieddad]

@agnivade No worries. I understand what you are saying.

@ianlancetaylor You keep saying "What is it that you want" despite me giving specific examples. I'll just write my own package and use that.

Thanks everyone!

[cespare]

@happilymarrieddad Ian is asking what you want because you haven't described the behavior you're looking for. Yes, you've given an example, but it's not possible for us to extrapolate from the example to a precise definition. I've read this thread several times now and I also don't know what your preferred meaning of "subtract a month" is.

To put it another way, you'd need to describe why subtracting a month from March 30 should give February 28, just like Agniva and Ian have explained why AddDate behaves as it does. (And what should subtracting a month from March 31 give? What about March 28 and March 29?)