Calculate Number of Business Days between Two Dates

function GetBusinessDays(startDate, endDate)
{
    if (startDate != null && endDate != null)
    {
        var cycletime = (Math.abs(endDate - startDate) / 86400000) + 1;
        var startday = startDate.getDay();
        var x = startday; // day of the week
        var y = 0; // number of business days for output
        var z = 0; // augment up to the total number of days

        while (z < cycletime) {
            if (x > 0 && x < 6) 
            {
                y++;
            }
            x++;
            z++;
            if (x > 6)
            {
                x = 0;
            }
        }

        return y;
    }
    return null;
}