Working with dates and time in WIX Code?

Try this

var date = new Date(tt);
    var newdate = new Date(date);

    newdate.setDate(newdate.getDate() + 3); // Adding three days
    
    var dd = newdate.getDate();
    var mm = newdate.getMonth() + 1;
    var y = newdate.getFullYear();

    var someFormattedDate = mm + '/' + dd + '/' + y;