further to mcnoodles answer, on some mechanical timers you put the little pegs in to set the on times, and on other you put them out to set the on time, it may say on your timer, or on a little drop dow flap which way you have to set the pegs, or it shoud say on your instruction book.
if you have no book go to baxi's website and email them and ask them for a copy of the manual for your boiler and clock, they are always very useful anyway if you call out a heating engineer for any repairs.