Confusing with commit time usage in logical decoding

Dear postgresql general mailing list,

I am currently using the logical decoding feature (version 9.6 I think
as far as I found in the source, wal_level: logical,
max_replication_slot: > 1, track_commit_timestamp: on, I am not sure
whether this will help or not).
Following the online documentation, everything works fine until I input

SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-timestamp', 'on');

I always got 1999-12-31 16:00 as the commit time for arbitrary
transactions with DML statements.
After several tries, I realize that the txn->commit_time returned was
always 0.
Could you help me by indicating me what could be wrong in my case? Any
missing parameters set?

Thank you in advance,
Kind Regards,
Weiping