You can :) You set a date and time for a tweet to be sent (as well as which Twitter accounts for it to be sent across), and then TweetingMachine waits until that time to send it to Twitter. Job done!
I suspect his application saves the datetime of when the tweet is to be made and simply submits it at that given time.
Got it in one, although before a user can schedule tweets they have to tell us what their local timezone is. So that, plus some funky timezone fun inbetween :)