public static TimeLock ofTimestamp(Instant time) {
        long secs = time.getEpochSecond();
        if (secs < THRESHOLD)
            throw new IllegalArgumentException("timestamp too low: " + secs);
        return new TimeLock(secs);
    }
@Test(expected = IllegalArgumentException.class)
    public void ofTimestamp_negative() {
        LockTime.ofTimestamp(Instant.EPOCH.minusSeconds(1));
    }