Calendar change is not altering finish time

Hi,

I have imported a contractor schedule and in order to get the logic to work correctly between theirs and our schedules, I have been required to alter their calendar to align with ours. the working hours are exactly the same (8 hours per day) but their calendar has a 1 hour lunch break meaning they finish at 17:00 and not 16:00 like ours.

When I update their activities with our calendar, the durations remain the same (again, both 8 hour days) but the activity finish times remain at 17:00. the only way I get around this is by manually reducing the remaining duration of an activity by 1 hour (which then changes the finish tome to 15:00 strangely) and then adding an hour back on so that it settles at 16:00. This can't be the best way to deal with this.