Formula calculating Time is not working if any input cells remain blank.

I have created a excle spreadsheet using 6 data entry cells to record the time in or out. I want to give the option of six in and outs and when each of these cells are used the time calculation formula works fine however if the employee only needs to use 2 or 4 of the input cells then my time calculation formula is not working. I don't know if it makes a difference or not but the input cells are data validated and are conditional upon the preceeding entry.

Julie