gototp.go
func (totp *TOTP) FromNow(periods int64) int32 {
	period := (time.Now().Unix() / int64(totp.Period)) + int64(periods)
	return totp.ForPeriod(period)
}
func (totp *TOTP) FromNow(periods int64) int32 {
	period := (time.Now().Unix() / int64(totp.Period)) + int64(periods)
	return totp.ForPeriod(period)
}
// Return the Time Based One Time Password for the time-period
// that is Now + the given periods.
// This is useful if you want to provide some flexibility around the acceptance
// of codes.
// For instance, you might want to accept a code that is valid in the current
// period (FromNow(0)), or that was valid in the previous period (FromNow(-1)) or that
// will be valid in the next period (FromNow(1)).
// This means that every code is therefore valid for 3 * totp.Period.