It allows you to predate your emails and automatically mark them read or unread in the recipient's mailbox. They just keep getting better and better over at Google.

Apparently you can use it an unlimited number of times every time you send an email, a maximum of ten times. Of course all the other gmail-wannabes will probably be following closely behind with their own, lackluster ripoffs, but here is the original.

Read more about it here: http://yuarel.com/gmail_custom_time