My CH Controller will not start at the preset time?

My Potterton 2000 programmer/Controller will not start at the preset time of 0730 but does so about 2 hours earlier - wasting gas! The Controller is about 20 years old. It switches off at the preset time of 2130. I have checked this fault over the last 5 days. Is there anything that I can do to restore it's starting at the correct time please? Regards, Fred Butcher