Most appliance's made by stoves has this problem if the timer is made by the same manufacturer. the stoves help line, in our hour of need could not help us as my wife and i have experienced, HHHMMmmm, to rectify, unplug or switch the appliance off at the Mains Supply and leave for 10 mins, holding two or three buttons at one time wont help. But allowing power drain to the timer will help to correct itself or will allow it to go into default mode which will then allow you to reset the clock display as required. it may take a second try but it should reset.
if you are not confident get help.
hope this helps.
Owen,, January 2012