How to show an annotated tag's timestamp?

["Dirk Süsserott" <newsletter@dirk.my1.cc>]

I'd like to display *when* a certain annotated tag was
created, i.e. its timestamp. I know two ways but am looking
for a third one:

Gitk: When clicking the yellow tag item (called 'testtag'
in my case) it says:
-------------
object 9a04a26d3e5ea09be32934f50020d0286ebe7325
type commit
tag testtag
tagger <tagger's name> <tagger's email-address> 1218568300 +0200
-------------
Well, I can convert 1218568300 into some human readable string,
but that's not convenient.

So I tried 'git show testtag'. It says:
-------------
(*) tag testtag
(*) Tagger: <tagger's name> <tagger's email-address>
(*) Date:   Tue Aug 12 21:17:17 2008 +0200
(*)
(*) testtag
commit 0a62b3380d21bf65270490619da4a2efd6d2ca7a
Author: ...
Date:   Thu Apr 24 18:49:38 2008 +0200
[...]
-------------

... followed by the whole commit message and diff of the
commit that's pointed to by the tag. That's too much for
my purpose.

Is there a way to output only when and by whom the tag
was created (incl. the tag's messsage), i.e. the lines
above marked with (*)?

AHA,
   Dirk