When you track vacation in hours, you are taking a percentage of the vacationable hours the employee worked and accruing it. Because it accrues in hours and not dollars, it doesn't convert into money until you actually pay it out. For example, if an employee earned $1000.00 for 80 hours worked for pay run 1 accruing vacation at 4%, then the employee has accrued 3.2 hours of vacation. Then, for pay run 2, the employee got a raise and now earns $1500.00 for the same 80 hours worked. They are still accruing at 4% and for pay run 2, they have still earned 3.2 hours of vacation. Whenever the employee decides to take their vacation, the hours are calculated into dollars using the rate in effect at the time they are taking the vacation not the rate it was associated with when it was accrued.