It's an error on Line 1163 of ImapSync:
Code:
$d = UnixDate(ParseDate($d), "%d %b %Y %H:%M:%S %z");
Should be
Code:
$d = UnixDate(ParseDate($d), "%d-%b-%Y %H:%M:%S %z");
to be RFC compliant.
After making that change, it works like a charm.
Seems the previous versions just send the date on to the destination in the same (hopefully RFC compliant) format the source gave it to them, but the new version parses it and reformats it. (But not quite to RFC standards.)