[javascript] How do I get Month and Date of JavaScript in 2 digit format?

$("body").delegate("select[name='package_title']", "change", function() {

    var price = $(this).find(':selected').attr('data-price');
    var dadaday = $(this).find(':selected').attr('data-days');
    var today = new Date();
    var endDate = new Date();
    endDate.setDate(today.getDate()+parseInt(dadaday));
    var day = ("0" + endDate.getDate()).slice(-2)
    var month = ("0" + (endDate.getMonth() + 1)).slice(-2)
    var year = endDate.getFullYear();

    var someFormattedDate = year+'-'+month+'-'+day;

    $('#price_id').val(price);
    $('#date_id').val(someFormattedDate);
});