cannot catch changed values of fields

Dear Ladies and Gents,
i am having 2 datepicker fields (Before Add To Cart Button) which works fine.
But if the valdiation had an error the “Add TO CART” button is inactive, no matter what i click.
So i wrote a small jQuery which disables the “ADD TO CART” button out if there are errors:

var idevArrival = jQuery("input[name=arrival]").val();
var idevDeparture = jQuery("input[name=departure]").val();

if (idevArrival == '' || idevDeparture == '') {
	alert('Empty values')
	jQuery('.single_add_to_cart_button').css( "display", 'none' );
}

But i am not able to catch changes on the 2 date fields to bring the button back.
I tryed with the jQuery “change” function but is doesn’t do the job:

    jQuery('input[name=arrival]').change(function(){
        alert("The text has been changed.");
    });

Any (quick) help is highly appreciated.
Many thanks in advance,
Joe