OK so a couple of days ago I thought I proved that time travel was not economically viable. Go trust Google to prove me wrong. What is more, the innovation occurred right here in Australia in its Sydney R&D centre. (No one could have predicted that.)
Click here to try Google gDay. It is only in beta but it works exactly as expected.
Also, on manipulating time. If you use Gmail, try Custom Time. Never need a late excuse again!