The length of the payroll period is controlled in the Payroll menu on the Payroll Assignments Calendar .

Search by company and budgetid.

Change the number of pay periods to the number of working days per month, for example in given year January might have 25 working days and February 20 days. 

After saving the calendar you will need to go back to Payroll Expense Definitions and click the "recalculate payroll trx" button.