Your request was fun to work out. Attached is a spread sheet that works pretty well. Column C has all the logic.
There is a readme on the second sheet.
Notes:
1. Accrual dates must be entered to get benefit
2. After inserting a new line, the "Current Balance" summation will break until the gap is filled. This can be repaired by filling, either down, up, or by cut and paste.
3. You can literally grab the bottom row, fill down, then delete any rows not desired.
Date HOURS SPENT Balance Comment
0 << Starting Balance
8/2/2008 0
8/15/2008 5 Time Accrued
8/20/2008 5
8/29/2008 5
8/30/2008 5
8/31/2008 10 Time Accrued
9/1/2008 8 2 Planned day off.
9/2/2008 2
9/15/2008 7 Time Accrued
9/16/2008 7
9/17/2008 7
9/29/2008 7
9/30/2008 12 Time Accrued
10/1/2008 12
Edit: Readme should say...
If you choose to move columns A or C, you will need to adjust the offsets in the formulas.