missing time calculations

hi

 I have a "missing time" issue.  I am inputting start time, finish time, and then missing time. the work sessions are 4hrs and I need to have the missed times from the 4hrs.  I have the calculation to get the missing time but if there has been no start/finish  times inputted then I have the 4hrs left in the column.  from that column I am totalling the missing hours on a summary page, so, if there has been no start/finish inputs I need them to read 0.