Smart timestamps in Go: decode/encode JSON from Unix timestamp numbers or RFC3339 strings

main.go

// https://play.golang.org/p/wSZvxQbT3KY

package main

import (
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"text/tabwriter"
	"time"
)

// Timestamp is like time.Time, but knows how to unmarshal from JSON Unix timestamp numbers or RFC3339 strings, and
// marshal back into the same JSON representation.
type Timestamp struct {
	time.Time
	rfc3339 bool
}

func (t Timestamp) MarshalJSON() ([]byte, error) {
	if t.rfc3339 {
		return t.Time.MarshalJSON()
	}
	return t.formatUnix()
}

func (t *Timestamp) UnmarshalJSON(data []byte) error {
	err := t.Time.UnmarshalJSON(data)
	if err != nil {
		return t.parseUnix(data)
	}
	t.rfc3339 = true
	return nil
}

func (t Timestamp) formatUnix() ([]byte, error) {
	sec := float64(t.Time.UnixNano()) * float64(time.Nanosecond) / float64(time.Second)
	return strconv.AppendFloat(nil, sec, 'f', -1, 64), nil
}

func (t *Timestamp) parseUnix(data []byte) error {
	f, err := strconv.ParseFloat(string(data), 64)
	if err != nil {
		return err
	}
	t.Time = time.Unix(0, int64(f*float64(time.Second/time.Nanosecond)))
	return nil
}

func main() {
	t := Timestamp{Time: time.Now()}
	fmt.Println("Now:", t.Format(time.RFC3339Nano), "==", t.Unix())
	w := tabwriter.NewWriter(os.Stdout, 0, 0, 1, ' ', tabwriter.Debug)
	defer w.Flush()
	fmt.Fprintf(w, "%s\t%s\t%s\t%s\n", "INPUT", "PARSED TIME", "INPUT in RFC3339", "ROUNDTRIP TO JSON")
	for _, s := range []string{
		`1257894000`,
		`1257894000.45221`,
		`"2009-11-10T23:00:00Z"`,
		`"2009-11-10T23:00:00.45221Z"`,
	} {
		err := json.Unmarshal([]byte(s), &t)
		if err != nil {
			panic(err)
		}
		j, err := t.MarshalJSON()
		if err != nil {
			panic(err)
		}
		fmt.Fprintf(w, "%s\t%s\t%t\t%s\n", s, t.Format(time.RFC3339Nano), t.rfc3339, j)
	}
}