title: Why is git AuthorDate different from CommitDate?
body: <p>I looked in my git logs and found that the AuthorDate and CommitDate is slightly different for some of my commits. From the <code>git log --pretty=fuller</code> output:</p>
  <pre><code>commit 3a5912f90dc5227f308e99f95152fbee2301c59a
  Author:     &lt;hidden&gt;
  AuthorDate: Fri Jun 15 10:57:22 2012 +0800
  Commit:     &lt;hidden&gt;
  CommitDate: Fri Jun 15 11:14:37 2012 +0800
  </code></pre>
  <p>The Author and Commit is the same (me).</p>
  <p>How does this happen? I have been puzzled for days.</p>
  <p>There are more - it happened in 17 out of 341 commits:</p>
  <pre><code>+------------------------------+-------------------------------+
  | from_unixtime(authored_date) | from_unixtime(committed_date) |
  +------------------------------+-------------------------------+
  | 2012-06-15 10:57:22          | 2012-06-15 11:14:37           |
  | 2012-06-15 14:39:54          | 2012-06-15 14:48:57           |
  | 2012-06-19 12:28:21          | 2012-06-19 12:29:41           |
  | 2012-06-21 18:16:25          | 2012-06-21 18:28:48           |
  | 2012-06-26 17:30:54          | 2012-06-26 17:33:55           |
  | 2012-07-13 11:41:43          | 2012-07-13 11:42:17           |
  | 2012-07-13 11:56:02          | 2012-07-13 12:13:22           |
  | 2012-07-13 12:05:09          | 2012-07-13 12:12:24           |
  | 2012-07-12 18:38:49          | 2012-07-13 12:26:35           |
  | 2012-07-13 11:00:47          | 2012-07-13 12:25:15           |
  | 2012-07-16 14:10:54          | 2012-07-16 14:15:01           |
  | 2012-07-13 12:56:51          | 2012-07-16 13:49:48           |
  | 2012-07-16 14:10:54          | 2012-07-16 14:19:46           |
  | 2012-07-24 16:05:05          | 2012-07-24 16:05:48           |
  | 2012-07-24 17:42:58          | 2012-07-24 17:43:33           |
  | 2012-07-24 17:42:58          | 2012-07-24 17:45:18           |
  | 2012-07-26 16:55:40          | 2012-07-26 16:55:53           |
  +------------------------------+-------------------------------+
  </code></pre>