I reinstalled OE to no avail. Further investigations revealed that the messages being forwarded did NOT contain any date / time stamp of when the message was actually sent in the message header. So, when it was forwarded, OE added the current date.
Digging deeper, I discovered that these emails were created by the scanner application bundled with a Hewlett Packard desktop scanner and not by OE. With the app, the user is able to scan the documents and email them directly to a recipient in the OE address book. The sent message ends up in the OE sent items but the message headers have not date and time of when the message was sent; though there is a time stamp in the Sent message list column, Sent.
If you are experiencing the same issue, check the message headers: highlight the message in the list view, select Properties from the file menu, Details Tab, click on the Message Source button. You should see an entry beginning with 'Date:'. If its missing then you know the problem is NOT with OE.
So, there is really no fix for this (other than getting HP to fix their app). If its important to you then you just have to remember to correct the date and time when forwarding the message.
Hope this helps someone :)