JavaScript For date Comparison Microsoft CRM

JavaScript For date Comparison with Error Message MSCRM

function ValidateDate(executionContext) {
    //debugger;
    var formContext = executionContext.getFormContext();
    if (formContext !== null) {
        var today = new Date();
        var Startdate = formContext.getAttribute('new_dateandtime').getValue();  //Date Schema Name
        if (Startdate > today) {
            alert("Date should not be past date");
            formContext.getAttribute('new_dateandtime').setValue(null);
        }
        else {
            formContext.getAttribute('new_dateandtime').setValue(Startdate);