Few years ago I flew back from the US during the week where the time difference was 4 not 5 hours and the flight was still scheduled to arrive at LHR at its usual time and they adjusted the departure time at the origin airport to cover it.
But I guess it’s not always possible to do that.
And yes as these dates are known about a long time in advance (unless you’re I think Morocco and you change your mind at the last minute) the airlines adjust their schedules for it.