
This is simple: I mean should be simple to implement :-)
These are mobile messages that are typed and set to sent at a future date (just like messages in your MS Outlook that can be sent at a future date). Could be handled by the phone application itself the way it remembers the reminders etc. Or by the server. The message sent can be stored at the server and later sent by server at the specified date.
The better solution would be to implement this feature at the application level so that a user may edit too if desired before the send-date.