Using Google's revolutionary "e-flux capacitor", you can "Just click 'Set custom time' from the Compose view. Any email you send to the past appears in the proper chronological order in your recipient's inbox. You can opt for it to show up read or unread by selecting the appropriate option."
Very handy indeed! Now on Saturday when I remember my dad's birthday was Friday, I can send him an e-mail and using Custom Time, have it show up in his Inbox like it arrived on his birthday! I can even "mark as read" so he thinks he looked at it and forgot!
For more information on this feature, click here.
Google, what will you think of next? By the way, what's the date today?