stopwatch.go

package main

import (
	"fmt"
	"time"
)

func main() {
	start := time.Now()
	go func() {
		for {
			elapsed := time.Since(start)
			fmt.Printf("\r%s", fmtDuration(elapsed))
			time.Sleep(time.Millisecond * 100)
		}
	}()
	var enter string
	fmt.Scanln(&enter)
}

func fmtDuration(d time.Duration) string {
	d = d.Round(time.Millisecond)
	h := d / time.Hour
	d -= h * time.Hour
	m := d / time.Minute
	d -= m * time.Minute
	s := d / time.Second
	d -= s * time.Second
	ms := d / time.Millisecond
	return fmt.Sprintf("%02d:%02d:%02d.%03d", h, m, s, ms)
}