Accrual Question

Everyone I need help. We have employees in our organization that are hourly that receive a bonus at the end of the year. Bonuses are based on base salary only, but some hourly employees work have shift differentials, where they may be paid, $25 for a few weeks, then back down to $21 for the month. I cannot for the life of me figure out how to do this. Does anyone have an excel example of this?

Thanks in advance.

Mike