AddDate return error result

[LouisYuBin]

Please answer these questions before submitting your issue. Thanks!

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

go 1.8

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

ubuntu kylin 16.04 LTS

What did you do?

`package main

import (
"time"
"fmt"
)

func main() {

userTime, err := time.Parse("2006-01-02", "2017-12-31")
if err != nil {
	fmt.Println(err)
}
lastMonthTime := userTime.AddDate(0, -1, 0)

fmt.Println(lastMonthTime)
lastMonthMonth := int(lastMonthTime.Month())
fmt.Println(lastMonthMonth)

}
`

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

2017-11-30 00:00:00 +0000 UTC
11

What did you see instead?

2017-12-01 00:00:00 +0000 UTC
12

[ianlancetaylor]

This is behaving as documented. Subtracting one month from "December 31" produces "November 31". Since there is no such date, it is normalized to "December 1". See https://golang.org/pkg/time/#Time.AddDate .