How do I set it for on at dusk and off at a specific time, not so many hours after dusk?

the photo cell the way i understand it is only to keep the info in the timer itself when either the timer is un plugged.or the power goes out.

I don't think it can be programed for both photocell and timer it's one or the other. Figure out what time it is dark and program that and the time you want it to go off.