西暦→和暦変換

jpn_year.go

package main

import (
	"errors"
	"fmt"
	"time"
)

var (
	asiaTokyo *time.Location
	eras      []JapaneseEra

	ErrNotImplemented = errors.New("not implemented yet")
)

type JapaneseEra struct {
	Name string
	Date time.Time
}

func init() {
	asiaTokyo, _ = time.LoadLocation("Asia/Tokyo")
	eras = []JapaneseEra{
		{Name: "令和", Date: time.Date(2019, 5, 1, 0, 0, 0, 0, asiaTokyo)},
		{Name: "平成", Date: time.Date(1989, 1, 8, 0, 0, 0, 0, asiaTokyo)},
		{Name: "昭和", Date: time.Date(1926, 12, 25, 0, 0, 0, 0, asiaTokyo)},
		{Name: "大正", Date: time.Date(1912, 7, 30, 0, 0, 0, 0, asiaTokyo)},
		{Name: "明治", Date: time.Date(1868, 9, 8, 0, 0, 0, 0, asiaTokyo)},
	}
}

func convert(d time.Time) (string, int, error) {
	for _, era := range eras {
		if d.After(era.Date) || d == era.Date {
			return era.Name, d.Year() - era.Date.Year(), nil
		}
	}
	return "", 0, ErrNotImplemented
}

func main() {
	now := time.Now()
	name, year, _ := convert(time.Now())
	fmt.Printf("%v => %s%d年", now.Format("2006/01/02"), name, year)
}

https://gist.github.com/mkfsn/2c96cc81376a42b119cd9f05da817857#file-jpn_year-go-L34
Change to !era.Date.Before(d)?

https://gist.github.com/PichuChen/e1929fb09f1848b60f061294cc97626a

Test failed