Help with PivotTable Time Calculations

Hello All,

I'm using Excel 2007 and new to this PivotTable thing. I've attached a sample spreadsheet with timestamp data on some medical procedures (the actual one contains thousands of records!). I have created a PT to tabulate the number of procedures that were discharged after 16:30, reflecting overtime hours for our staff. Basically I have sorted out all the procedures with a discharge time >= 1630. Simple enough. I am having trouble figuring out how to tabulate the total overtime hours (sum of the difference between discharge time and 16:30) for all these procedures that went over. Most of it seems to be error since most of the differences are negative times! Any Ideas?

Thanks,
awke