Night feed is done usually after all the boarders have left (around 10PM). Morning feed is done around 5AM. Between those two times I don't think it's appropriate to show up to the barn, unless you are leaving for a show/clinic very early in the morning, or returning from a show/clinic very late.
They expect also that the last ones to leave will lock up and turn out the lights.
Barns that I have boarded at with hours have all been able to make exceptions for jobs, school hours, etc.. Mainly I think hours are mainly to deter inconsiderate behavior. Whenever I was at the barn "after hours" I would always help with final chores, sweep, help close up, etc..