$ git show-ref --tags | grep my_timestamp_tag
nonexistent_commit_hash refs/tags/my_timestamp_tag

$ git log | grep nonexistent_commit_hash
$

$ git reflog | grep nonexistent_commit_hash
$

$ git checkout nonexistent_commit_hash
Note: checking out 'nonexistent_commit_hash'.

You are in 'detached HEAD' state.
...
HEAD is now at hash_for_the_actual_commit... message_for_the_actual_commit

$ git checkout my_timestamp_tag
Note: checking out 'my_timestamp_tag'.

You are in 'detached HEAD' state.
...
HEAD is now at hash_for_the_actual_commit... message_for_the_actual_commit

$ git show my_timestamp_tag`
tag PXEPLAT_20150303_162652
...
message_for_my_timestamp_tag

commit hash_for_the_actual_commit
...