Immense power needed for time travel means they can only send people backwards and forwards a maximum of 30 years. Where's my cheque gift certificate, Amazon?
edit: just thought, you could hop backwards in 30 year increments once the time machine has powered up again. Okay, the time machine uses a special (time?) crystal which is so volatile, it can only be used once. You can only take two with you because of time travel health and safety regulations, one to go back, one to come back. If you take more than three special (time?) crystals, you run the risk of the special (time?) crys