I had found the month in payslip.name .Then i simple tried that is month in that payslip name or not but in this solution we have to force user not to change payslip name pattern . I had changed the payslip name to read only to achieve this

     datej = str(payslip.date_to)
    datek = str(payslip.date_from)
    dj = int(datej[-2:])
    dk = int(datek[-2:])
    working_days=dj - dk
    month=0
    if "January" in payslip.name: month= 31
    if "February" in payslip.name: month= 28
    if "March" in payslip.name: month= 31
    if "April" in payslip.name: month= 30
    if "May" in payslip.name: month= 31
    if "June" in payslip.name: month= 30
    if "July" in payslip.name: month= 31
    if "August" in payslip.name: month= 31
    if "September" in payslip.name: month= 30
    if "October" in payslip.name: month= 31
    if "November" in payslip.name: month= 30
    if "December" in payslip.name: month= 31 
    one_daySalary=contract.wage/month 
    salary=one_daySalary*working_days
     result = salary