Wage Type with a specific day

Dear Experts;

I need to create a bonus that will be recorded with a number of days value. This number of days will be converted to an amount from the basic pay. The issue is, they want it to be calculated based on the amount that is recorded in the Basic Pay wage type with date 30.06.2015.

Whenever the date of this bonus, it should be calculated based on the amount of the basic pay in 30.06.2015. Can you help me know how to make that condition.

Thanks in advance.

Best Regards;

Lobna