Much thanks to you both. Niek's answer was what I needed to fix my problem
and finish deveoping the timecard. I now have a daily timecard (which, when
printed, can fit on one page) on which the user enters "in" time
(CTRL+SHIFT+;), task description, and department code. When the task is
completed, the user enters "out" time, which then becomes the "In" time for
the next task. The spreadsheet imediately updates total daily time for each
department as well as total for the day, and updates a weekly summary that
also reflects total by department and week, and compares those totals to
budgeted times.

All I needed was Niek's formula, and about 4 hours. I started using the
timecard today, and it seems to work fine.