Golang で日付の1ヶ月前と30日前を表示してみたメモ

なんで3月の1ヶ月前がそうなるのという気持ち

package main

import (
    "fmt"
    "time"
)

func main() {
    dateList := []string{"2022-05-31", "2022-04-30", "2022-03-31", "2022-02-28", "2022-01-31"}

    fmt.Println("# 1ヶ月前")
    for _, d := range dateList {
        t, _ := time.Parse("2006-01-02", d)
        fmt.Printf("%v -> %v\n", d, t.AddDate(0, -1, 0).Format("2006-01-02"))
    }

    fmt.Println("")

    fmt.Println("# 30日前")
    for _, d := range dateList {
        t, _ := time.Parse("2006-01-02", d)
        fmt.Printf("%v -> %v\n", d, t.AddDate(0, 0, -30).Format("2006-01-02"))
    }
}

結果

# 1ヶ月前
2022-05-31 -> 2022-05-01
2022-04-30 -> 2022-03-30
2022-03-31 -> 2022-03-03
2022-02-28 -> 2022-01-28
2022-01-31 -> 2021-12-31

# 30日前
2022-05-31 -> 2022-05-01
2022-04-30 -> 2022-03-31
2022-03-31 -> 2022-03-01
2022-02-28 -> 2022-01-29
2022-01-31 -> 2022-01-01