Only you know when you will get home
Only you know when two hours before that is
Only you know what time it is when you set the timer
It's a delay timer - so - if you set it at 07:00 in the morning just before going out - and you get back at 18:00 after a busy day - and you want the oven to begin cooking two hours before this - you need to do a bit of basic maths.
16:00 start and 07:00 now - 16 minus 7 = 9
So you set a 9 hour delay
Of course - I could be wrong - perhaps you just set it to 16:00 the time you want cooking to begin.
The book should tell you which.
Good luck...enjoy your roast dinner...
Peccavi
May 2010