Employee database

Hi,
I have to design a databse for our human resources office.

They need a database to keep track of employee overtime hours.
Overtime hourly rate depends on the employee's position.
Over the year, the hourly rates could be updated (rate increase) and the position itself could change (promotion).
When one, or both, of these events occurs overtime worked form the beginning of the year must be recalculated with the new rates.
Better, a new amount must be calculated with the rate difference (current - previous) and the overtime hours.

How to model this?