Movie Times Calculation?

I work for a major theatre chain an no one (inclduing the help desk) can figure
out how to make a show start/end time schedule calculate the ending time. For
example A3 would list the lenght of the show in minute, A4 (5,6, and so on)
would list the start time(s). We want B4 (5,6, etc) to list the end time.
We have a formula now but it won't work with shows that jump over 12 noon or 12
midnight without manually going into the formula and adding or subtracting 12.

Can anyone help or direct me to a solution source?