code: private String getShardKey(long messageTime, int modShard) {
          long timePartition;
          if (metadata.getPartitionDuration() != null)
              timePartition = (messageTime / metadata.getPartitionDuration()) % metadata.getPartitionCount();
          else
              timePartition = 0;
          return getName() + ":" + timePartition + ":" + modShard;
      }
docs: Return the shard for this timestamp
  @param messageTime
  @param modShard
  @return