changing calculation result to negative

I have a custom calculation script in an Acrobat form field:

var myFormat = "mm/dd/yyyy"

var startDt = this.getField("PolicyEffectiveDate").value
var endDt = this.getField("ChngDate").value

var startDate = util.scand(myFormat,startDt);
var endDate = util.scand(myFormat,endDt);

var xdays = Math.round(Math.abs((startDate.getTime() - endDate.getTime())/(86400000)))

event.value =  ((365 - (xdays)) / 365).toFixed(3) * (this.getField("AnnualPremium2").value)

How would I change the resulting calculation total to negative?

Thanks in advance for any help!