time: Wrong `time.AddDate` calculation if months' last date is 31

[ihchristianto]

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

$ go version
go version go1.19.3 darwin/arm64

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
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/immanuel.christianto/Library/Caches/go-build"
GOENV="/Users/immanuel.christianto/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/immanuel.christianto/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/immanuel.christianto/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/Cellar/go/1.19.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.19.3/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.19.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/immanuel.christianto/Workspace/go-test/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/jc/_p3v_n897w36p97jwnm4splr0000gp/T/go-build1081775705=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://go.dev/play/p/CHijIVQtJS4
Try subtracting one month from any month whose last date is 31.
oneMonthBefore := now.AddDate(0, -1, 0)

What did you expect to see?

Now: 2022-01-31 20:20:20 +0700 WIB, One Month Before: 2021-12-31 20:20:20 +0700 WIB
Now: 2022-03-31 20:20:20 +0700 WIB, One Month Before: 2022-02-28 20:20:20 +0700 WIB
Now: 2022-05-31 20:20:20 +0700 WIB, One Month Before: 2022-04-30 20:20:20 +0700 WIB
Now: 2022-07-31 20:20:20 +0700 WIB, One Month Before: 2022-06-30 20:20:20 +0700 WIB
Now: 2022-08-31 20:20:20 +0700 WIB, One Month Before: 2022-07-31 20:20:20 +0700 WIB
Now: 2022-10-31 20:20:20 +0700 WIB, One Month Before: 2022-09-30 20:20:20 +0700 WIB
Now: 2022-12-31 20:20:20 +0700 WIB, One Month Before: 2022-11-30 20:20:20 +0700 WIB

What did you see instead?

Now: 2022-01-31 20:20:20 +0700 WIB, One Month Before: 2021-12-31 20:20:20 +0700 WIB
Now: 2022-03-31 20:20:20 +0700 WIB, One Month Before: 2022-03-03 20:20:20 +0700 WIB
Now: 2022-05-31 20:20:20 +0700 WIB, One Month Before: 2022-05-01 20:20:20 +0700 WIB
Now: 2022-07-31 20:20:20 +0700 WIB, One Month Before: 2022-07-01 20:20:20 +0700 WIB
Now: 2022-08-31 20:20:20 +0700 WIB, One Month Before: 2022-07-31 20:20:20 +0700 WIB
Now: 2022-10-31 20:20:20 +0700 WIB, One Month Before: 2022-10-01 20:20:20 +0700 WIB
Now: 2022-12-31 20:20:20 +0700 WIB, One Month Before: 2022-12-01 20:20:20 +0700 WIB

Hint: instead of going back 1 month, the result seems to subtract the Now date by the number of days in the previous month. Ex, 31 March goes back to 3 March, because February has 28 days. 31-28 = 3

[ZekeLu]

It works as intended according to the doc https://pkg.go.dev/time#Time.AddDate

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.

[bcmills]

It works as intended according to the doc https://pkg.go.dev/time#Time.AddDate

Agreed: this is working as documented.

“One month before” March 31st would be February 31st, which is not a valid date. There are two possible “reasonable” behaviors: clamp to the last valid date of the month, or compute “February 31st” as 30 days after February 1st. time.AddDate does (and is documented to do) the latter.

[bcmills]

if I subtract and add again the result will not be the same.

That would also be the case if AddDate clamped to the last day of the month: if you say that “one month before” March 31st is February 28th, well, “one month after” February 28th is March 28th, not March 31st.

[ianlancetaylor]

See earlier issue #31145.