OverTime Calculation Error

Hi Friends, I have a requirement where the Overtime is calculated as follows :

Basic Salary divided by No of Calender Days in a month= 1 Day Salary

Then 1 Day Salary divided by No of Working Hours on a Normal Working Day = Per Hour Salary

Then No of Hours of Overtime stored in 2010 multiplied with 2 Times the Per Hour Salary = Overtime Amount.

I have wrote a PCR for Overtime Calculation as calculation of OT varies from Employee Group to Employee Group.

In the PCR to get the exact No of Calender Days each month I have used variable GKSOLL and it is working fine in all scenarios except one where employee joins or leaves mid month. In such scenario GKSOLL stores the values of No of Calender Days employee was available. Is there any other variable which will always given the No of Calender Days of the respective month which can be used in this PCR. Please advise.

Regards,

SP