Right now you are able to see both the e-mail's timestamp and the signature's timestamp. However, Enigmail does not try to cross check them. A warning should be displayed if they're too far apart, since it indicates a potential problem.
For example, if you sent a message whose meaning was mostly in the subject, such as:
Date: 01/01/2010 Subject: Meet me at John's tommorow Cya! -- Dave.
Then an attacker would be able to send to anyone else a message:
Date: 21/12/2013 Subject: Sell all your stock RIGHT NOW! Cya! -- Dave.
No warning would be displayed on current Enigmail behaviour.