So now you can send Birthday greetings to your Mum for 2005 and bamboozle your boss by saying that you sent him that report you forgot to do before Christmas! Unfortunately you can only send emails back until April 1, 2004, the day that Gmail was launched.
Gmail have also limited the each user to just ten pre-dated emails per year, so you'll have to choose carefully. The fear is that an unlimited supply of custom time emails would cause people to lose faith in the accuracy of time making the feature useless.
Google Gmail Custom Time looks set to change the nature of modern communication for ever.