Excel time/duration spreadsheet
 

I am struggling with a spreadsheet listing 50 presentation start and stop
times and duration. I need a formula(s) so when a duration for a
presentation changes, the entire spreadsheet's start and stop times change as
well. I have used "=IF(A4B4,B4+1-A4,B4-A4)" to calculate the duraction
between start and stop, but cannot figure out how to get the rest of the
spreadsheet to undate when changed. Any help will be much appreciated!!