It's wrong in the current implementation. The current implementation uses it as the start of the slice [1][2]. I intend to fix it after moving to the new CassieSpanStore.

1) https://github.com/twitter/zipkin/blob/master/zipkin-cassandra/src/main/scala/com/twitter/zipkin/storage/cassandra/CassandraIndex.scala#L147
2) https://github.com/twitter/cassie/blob/master/cassie-core/src/main/scala/com/twitter/cassie/ColumnFamily.scala#L148