keeping a running time for tv program
 

I would like to use Excel for a television program director's rundown
sheet. Is there a simple formula that would show running time by
segment from the start of the program to the end? In other words, the
program would start at 00:00:00 and end at 30:00:00. I would like one
column to keep a running time total for each element of the program.
Thanks for your help!