Some time we need to calculate number of days between two dates in crm, we can easily do this using below JS script

var d1=crmForm.all.FirstDate.DataValue;
var d2=crmForm.all.SecondDate.DataValue;
var DayValue=1000*60*60*24;

alert(Math.ceil((d2.getTime()-d1.getTime())/DayValue));

Update for Dynamics 365

var d1=Xrm.Page.getAttribute(“FirstDate”).getValue();
var d2=Xrm.Page.getAttribute(“SecondDate”).getValue();

var DayValue=1000*60*60*24;

alert(Math.ceil((d2.getTime()-d1.getTime())/DayValue));