Date calculation Issue

Hi, 

When I calculate the End Date from the Start Date  the start date is also set to End date.

This is the code i used to set the end date but once i set the end date the start date value is also set to the end date value.

function calculateTenancyEnd() {

   var Month = Xrm.Page.data.entity.attributes.get("month").getValue();

   var StartDate = Xrm.Page.data.entity.attributes.get("startdate").getValue();

   if (StartDate != null) {

       if (Month != null) {          

           StartDate.setMonth(StartDate.getMonth() + Month);

           StartDate.setDate(StartDate.getDate() - 1);

           Xrm.Page.data.entity.attributes.get("enddate").setValue(StartDate);          

       }

   }

}