|You need to provide more information.|
We don't know what data you have available to use for the calculation. A start date in one cell, an end date in another? The amount of the bonus in a 3rd cell?
Keep in mind that we can't see your spreadsheet from where we're sitting, so there's no point in trying to offer a solution that isn't going to work for you.
Click Here Before Posting Data or VBA Code ---> How To Post Data or Code.