I am not sure this answers your question or if this is what you have already, but I figured I would give it a shot.
I have a time sheet that I use to record times I work on a project. I have a start time and and end time. Both of them are Formatted using the Time 13:30:55 Format. Then I just subtract the two and I have the time I spent on a given day or period (also using the Time 13:30:55 Format). Finally I add up all of those times for my Total Hours. The Format I use on the Total Hours is Custom [h]:mm:ss;@ - the last selection under Custom.