How to call a shell script from awk ?

Shell Programming and Scripting
1 
BEGIN { account_no = substr($0,1,8)}
        { billdate = substr($0,13,20)}
        { billduedate = substr($0,21,28)}
        { billid = billid - 1}
        { billduedate = billdatecalc(billduedate - 1)}
         ...
END ....

How do I call the shell script billdatecalc(arguments ..) in an awk script ?
Please guide....

Thanks
Amruta

2 
"billdatecalc " billduedate - 1 | getline billduedate