So make them always based on data.
Suppose your hours worked table looks something like this:
Date,Hours
01/12/09,8
02/12/09,7.5
03/12/09,7.25
04/12/09,8
05/12/09,8
...and the manager looks at it and decides 39 hours is too much, he thinks it should be 38, just store another row with his...