Chickens seem to have a rather accurate internal clock.
Our automatic pop hole goes up at 5.30am, but Bella, our noisy hen, doesn't start calling to me until about 7.50am, because she knows I go out there with breakfast and to let them out on free range at about 8am. It's quite impressive really.
Bedtime does seem to be rather more flexible though. It rather depends on how interesting their day is turning out to be. If they're locked in their run at dusk, they'll go to bed earlier than if I've let them stay out on free range. (Our BO has been known to go to bed at 7.30pm if she's really bored in her run on a summer evening, whereas they should be roosting at about 8.45pm at this time of year in this part of the world.)