At her age I wouldn't be to concerned with a bed 'time' as such, just more interested in a bed time routine, they don't start working more towards a clock time until about 6 months. Next week it will probably be different again, its just to keep us mummies on our toes. If they don't want to sleep then you probably wont get them to.

DD2 is still changing her bed times, sometimes she goes down and stays down, other times wakes up and plays happily in her cot until she goes back to sleep and other times screams blue murder until someone gets her and she plays again till 10.30. Try to remember how much sleep she has gotten in the 24 hours, they tend to average out the same each day, so if they have longer day time naps they have less sleep during the night etc.