if exp == nil || exp.IsZero() || exp.Equal(timeSentinel) {
		return nil
	}
	dur := exp.Sub(time.Now())
	if dur <= 0 {
		return errors.New("unsupported expiration time")
	}
	condValues["DurationSeconds"] = []string{strconv.FormatInt(int64(dur.Seconds()), 10)}
	return nil
}