Gravity Forms calculate number days between 2 date fields

gistfile1.js

<script type="text/javascript">
	jQuery( document ).ready( function( $ ) {
		var ms_one_day = 1000 * 60 * 60 * 24;

		var form_id     = '1';
		var date_format = 'dd-mm-yy';

		var field_date_start  = $( '#input_' + form_id + '_1' );
		var field_date_end    = $( '#input_' + form_id + '_2' );

		var field_number_days = $( '#input_' + form_id + '_16' );

		var update_number_days = function() {
			var value_date_start = field_date_start.val();
			var value_date_end   = field_date_end.val();

 			var date_start = $.datepicker.parseDate( date_format, value_date_start );
			var date_end   = $.datepicker.parseDate( date_format, value_date_end );

			if ( date_start && date_end ) {
				// Convert both dates to milliseconds
				var ms_date_start = date_start.getTime();
				var ms_date_end   = date_end.getTime();

				// Calculate the difference in milliseconds
				var ms_difference = Math.abs( ms_date_start - ms_date_end );

				// Convert back to days and return
				var number_days = Math.round( ( ms_difference + ms_one_day ) / ms_one_day );

				if ( $.isNumeric( number_days ) ) {
					field_number_days.val( number_days );
					field_number_days.trigger( 'change' );
				}
			}
		};

		field_date_start.change( update_number_days );
		field_date_end.change( update_number_days );

		field_number_days.prop( 'readonly', true );

		update_number_days();
	} );
</script>