While there’s still currently no workaround for backdating an email (Professor, I swear I sent it to you yesterday. Check your emails again.), a Gmail plug-in for Chrome and Firefox called Boomerang can allow you to at least schedule emails to be sent at a later time/date.
Let’s put this to use in a practical sense:
You’re drunk, arrive home way later than you wanted, and decide you want to take a sick day tomorrow. Instead of waking up with a bad headache and composing a quick “I’m sick” email, you can write it while you’re still awake and schedule the email to be sent at 7:38. Now you can get some rest worry-free. Just make sure to double-check what you write. Boomerang does not currently support proofreading for drunk grammar or spelling.
via Laughing Squid