I am testing migration from a Solaris 9/dovecot system to Zimbra 5.0 on RHEL5. imapsync seems to work very well except for some occasional errors like:

Use of uninitialized value in concatenation (.) or string at /usr/bin/imapsync line 1340.
10791 BAD parse error: invalid date format
 at /usr/bin/imapsync line 1358
Couldn't append msg #28587 (Subject:[something]) to folder INBOX: 10791 BAD parse error: invalid date format
By occasional, I mean a few hundred such erroneous e-mails for the entire migration of 300000+ e-mails.

I tried finding the bad date headers, but can't see anyhting wrong with them. Here are two examples:

Date: Sun, 31 Dec 2006 17:18:22 -0800

Date: Mon, 31 Dec 2007 16:04:21 -0800
This led me to check the dates associated with more of the errors, and they are all December 31st!

Any ideas what is going on here?