Invalid date in IMAP messages
From MozillaZine Knowledge Base
If messages in your IMAP account have a 1/1/1970 date thats due to Thunderbird not knowing the messages time/date, and using the default value. The default value is the oldest possible value. Most users only see a 1/1/1970 date if they have a POP account with a badly corrupted folder, and it loses the Date: header in one or more message fragments.
There are two ways to specify the time/date in a IMAP message. The IMAP server can return a Date: header and/or return a IMAP INTERNALDATE message attribute. Most email clients (such as Outlook/Outlook Express) support the IMAP INTERNALDATE message attribute. Thunderbird only knows how to use the Date: header. If the IMAP server returns just the IMAP INTERNALDATE message attribute this could cause other email clients to display the correct time and date, while Thunderbird always displays 1/1/1970 .
There doesn't seem to be a Bugzilla bug report on this problem. This might be because most IMAP servers appear to return both.