// e.g. returns 00:00:25
func getTimeDiff(a, b time.Time) string {
	d := b.Sub(a)

	hour := int(d.Seconds() / 3600)
	minute := int(d.Seconds() / 60) % 60
	second := int(d.Seconds()) % 60

	return fmt.Sprintf("%02d:%02d:%02d", hour, minute, second)
}