Retirement monthly calculation

I am creating a formula for retirement in adobe acrobat but I am not getting correct results. 

=PMT(C17,C18,D16,-D13)

C17 is montly return rate
C18 is # of months until retirement
D16 is current savings
D13 is Amount needed in retirement 

var rate = Number(this.getField("C17").value);
var nper = Number(this.getField("C18").value);
var pv = Number(this.getField("D16").value);
var fv = -Number(this.getField("D13").value);

var pmt = (pv * rate) / (1 - Math.pow(1 + rate, -nper)) + (fv / Math.pow(1 + rate, nper));

if (this.getField("C17").value === "" || this.getField("C18").value === "")
event.value = "";
else
event.value = pmt;