Today you can use the new feature of Gmail ~ custom time. With custom time you will never be late for anything again. You can back date any email up to April 1, 2004. This way you will never miss another birthday or appointment without an explanation.
Check out the details here.
Here are some testimonials
By the way, if you missed it, this is their April Fool’s joke for 2008.
Last years April Fools joke from Gmail was pretty funny too! Paper copies of Email sent directly to you so you can back up all your emails (click here)