how to calculate a date based of another date and a dropdown box ?

Hi, I have this code that allow me to give a date based on another date :

    var cDate = this.getField("DateDebut18").value;
    if (cDate) {
    var d1 = new Date();
    d1 = util.scand("dd/mm/yyyy", cDate);
    d1.setFullYear(d1.getFullYear() + 1); // this adds one year
    d1.setDate(d1.getDate() - 1); // this subtracts one day
    event.value = util.printd("dd/mm/yyyy", d1);
    }

However, I need a code that allow me to obtain a date based on another date and the value of a dropdown box that have three options : 1 year, 2 years, 3 years. What is the code ? Thank you.