code: public Set<TtlBucket> getExpiredBuckets(long time) {
      return mBucketList.headSet(new TtlBucket(time - TtlBucket.getTtlIntervalMs()), true);
    }
docs: Retrieves buckets whose ttl interval has expired before the specified time, that is, the
  bucket's interval start time should be less than or equal to (specified time - ttl interval).
  The returned set is backed by the internal set.
  
  @param time the expiration time
  @return a set of expired buckets or an empty set if no buckets have expired