contract for the employee is not working properly

Hi,
i want to create a contract for the employee functions Driver,
Under Contract tab , in Salary i have selected Monthly gross wage as wage type and wage value is 2000.

In the contract tab i have given contract name as "temp contract"
and start date i have given as 12-20-2009 and endate as 12-30-2009

But when i tried to create a salary slip for this employee , using HumanResource ->payslip->Employee payslip ,

when i select the compute sheet button , it is displaying working days =23 , though i have mentioned the start date and
end date in the contract