Which dates 'git log --since= --after=' compare?

Hi,

I can see that 'git log --since= --after=' compares commit's dates,
not author's dates.How can I limit commits by Author's date?

$ git log --format="%aD %cD"

Mon, 5 Oct 2009 14:54:57 +0200 Thu, 8 Oct 2009 10:31:40 +0200
Mon, 21 Sep 2009 11:11:35 +0200 Thu, 8 Oct 2009 10:31:40 +0200
Mon, 21 Sep 2009 10:14:41 +0200 Thu, 8 Oct 2009 10:31:40 +0200
Wed, 16 Sep 2009 14:23:30 +0200 Thu, 8 Oct 2009 10:31:40 +0200
Wed, 16 Sep 2009 10:27:39 +0200 Thu, 8 Oct 2009 10:31:38 +0200
Wed, 16 Sep 2009 09:15:32 +0200 Thu, 8 Oct 2009 10:30:31 +0200
Wed, 16 Sep 2009 08:02:23 +0200 Wed, 16 Sep 2009 09:02:29 +0200

$ git log --format="%aD %cD" --since=2009-09-01 --until=2009-10-01
Wed, 16 Sep 2009 08:02:23 +0200 Wed, 16 Sep 2009 09:02:29 +0200

-- 
Daniel