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

$ go version
go version go1.15.4 windows/amd64

Does this issue reproduce with the latest release?

source code

func main() {
	date := time.Date(2020, 12, 31, 0, 0, 0, 0, time.Local)
	t := date.AddDate(0, -1, 0)
	fmt.Println("t:", t)
	strMonth := t.Format("200601")
	fmt.Println("month:", strMonth)
}

result

t: 2020-12-01 00:00:00 +0800 CST
month: 202012

area: china

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

$ go env

What did you do?

What did you expect to see?

What did you see instead?