Formula To Reset a Count Based On Cell Value

I've found a # of threads on this subject but have been unable to find the function I need.

I've got a spreadsheet (attached) to calculate vacation time, but I can't figure out how to make the used time reset on an anniversay date. For example, if Employee 1 has earned 80 hours of vacation but only used 40 by his 4 year anniversary date, how do I make unused time roll back to 80 the day of his anniversary?

Here's the structure: 2 Personal days given Jan. 1, then 2 more on July 1 to employees here at least 6 months. Those whose 6 months fall after or between are awarded 2 days on their anniversary then accrue as normal. Unused personal days are lost at the end of the year. Vacation time is 40 hours after 1 year, 80 hours at 3 yrs, then 120 hours at 5+. These are given strictly by anniversary date & are forfeited if unused by the next anniversary.

Thanks for your help!