ie they can and do alter flight times to suit them, not you.
EUJet have just emailed me to tell me that they have cancelled our booking on a 17.40 flight from Manchester to Manston on 26-5-05 and only substitute leaves at 09.00! This is just about OK, but they've also cancelled our return flight, which was due to leave at 16.05 on 30-5-05 and sole substitute is at 07.25!
I've no doubt their terms and conditions allow them to do this, but it does show that it isn't quite like flying with a big scheduled airline, though I think even they occasionally alter/cancel flight times-see "Air Canada=bastards!" thread.