GMail messages show wrong date after import - date

Question

I have created a Google Apps account and then important my old email messages from my own IMAP-based mail server. For that purpose I used imapsync.
There is some mails which show up fine when viewed in Thunderbird -- interestingly enough as well on the old IMAP server as on GMail. They correctly have the date of original sending at 2nd of December 2008. When viewing the full mail source ("original message") from GMail web interface, its headers read (XXX for anonymization in some places):
X-Account-Key: account6
X-UIDL: 10f6988675937085b6eXXX
X-Mozilla-Keys:
Return-Path: <XXX#t-online.de>
X-Flags: 0000
Delivered-To: GMX delivery to XXX#gmx.de
Received: (qmail invoked by alias); 02 Dec 2008 09:16:48 -0000
Received: from mailout09.t-online.de (EHLO mailout09.t-online.de) [194.25.134.84]
by mx0.gmx.net (mx086) with SMTP; 02 Dec 2008 10:16:48 +0100
Received: from fwd02.aul.t-online.de
by mailout09.sul.t-online.de with smtp
id 1L7RN9-XXX; Tue, 02 Dec 2008 10:16:47 +0100
Received: from localhost (Tti8HrZYYh01uPSGlzL36WA4trfZ2XXX#[172.20.101.250]) by fwd02.aul.t-online.de
with esmtp id 1L7RMr-XXX; Tue, 2 Dec 2008 10:16:29 +0100
MIME-Version: 1.0
Received: from 77.185.232.243:1675 by cmpweb16.aul.t-online.de with HTTP/1.1
(Kommunikationscenter V9-2-23 on API V3-3-19)
Date: Tue, 02 Dec 2008 10:16:29 +0100
Reply-To: "XXX#t-online.de" <XXX#t-online.de>
To: XXX#gmx.de
X-Priority: 3
X-UMS: email
X-Mailer: TOI Kommunikationscenter V9-2-23
Subject: XXX
From: "XXX#t-online.de" <XXX#t-online.de>
Content-Type: multipart/mixed;
boundary="=_71bfbbb1eca6e77241917c40d55c1078"
Message-ID: <1L7RMr-XXX#fwd02.aul.t-online.de>
X-ID: Tti8HrZYYh01uPSGlzL36WA4trfZ2Ib9MXXX#t-dialin.net
X-TOI-MSGID: 6a3e3035-aa28-XXX
X-GMX-Antivirus: 0 (no virus found)
X-GMX-Antispam: 0 (Mail was not recognized as spam)
X-GMX-UID: GcoGeJ9WIyd1gjzXXX
But in the overview lists in the web interface it shows as dated 7th of October 2011. This date might be the date of a previeous import (to my own IMAP server).
But I have no idea where Google takes that date from and how I can change this date to the date the message was really sent. Any ideas?

Answer 1

I found this worked for me -
Copy all mails via IMAP from GMail to a local/different account folder
Now delete all mails from GMail using their Web interface
Empty the GMail trash
Copy all mails back from local/different account to GMail
Source - http://productforums.google.com/d/msg/gmail/0PU1gQuXNi8/HGu6GRigerMJ