AdminClient ListOffsetsResultInfo/timestamp is always -1

When using AdminClient/listOffsets the resulting ListOffsetResultInfos appear to always have a timestamp of -1.

I've run listOffsets against live clusters with multiple Kafka versions (from 1.0 to 2.5) with both CreateTIme and LogAppendTime for message.timestamp.type, every result has -1 timestamp.

e.g. 

org.apache.kafka.clients.admin.ListOffsetsResult$ListOffsetsResultInfo#0x5c3a771
ListOffsetsResultInfo(} offset=23016, timestamp=-1, leaderEpoch=Optional[0])