long getExpirationTimestamp(final Duration ttl) {
    return clock.instant().plus(ttl).getEpochSecond();
  }
@Test
  void getExpirationTimestamp() {
    assertEquals((CURRENT_TIME_MILLIS / 1000) + 3600,
        pushChallengeDynamoDb.getExpirationTimestamp(Duration.ofHours(1)));
  }