How do i change this?

/*** Get the ISO week date week number */

Date.prototype.getWeek = function () {
    // Create a copy of this date object;
    var target  = new Date(this.valueOf());
    //var target  = new Date(this.getField("WeekBegin").value);

    // ISO week date weeks start on Monday;
    // so correct the day number;
    var dayNr   = (this.getDay() + 6) % 7;

    // ISO 8601 states that week 1 is the week;
    // with the first Thursday of that year.;
    // Set the target date to the Thursday in the target week;
    target.setDate(target.getDate() - dayNr + 3);

    // Store the millisecond value of the target date;
    var firstThursday = target.valueOf();

    // Set the target to the first Thursday of the year;
    // First set the target to January first
    target.setMonth(0, 1);
    // Not a Thursday? Correct the date to the next Thursday
    if (target.getDay() != 4) {
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
    }

    // The weeknumber is the number of weeks between the
    // first Thursday of the year and the Thursday in the target week
    return 1 + Math.ceil((firstThursday - target) / 604800000); // 604800000 = 7 * 24 * 3600 * 1000
}

var oDate = new Date(); // create date object;
var nWeekNumber = oDate.getWeek(); // get the week number for the date object;
//app.alert("For " + util.printd("ddmmyy", oDate) + " it is week " + nWeekNumber, 3, 0);

this.getField("WeekNumber").value = nWeekNumber;

Now I Want to be able to use the "weekbegin" field text box (ddmmyy) to work out the week number, but instead its using the current date.

What do I change to get it to use the "weekbegin" as the date instead of the current date it uses?