jquery-ui-cal.js

jQuery( function( $ ) {
	var start, end, nights;

	$( '.simpay-date-input' ).datepicker( 'option', 'onSelect', function() {
		startVal = $( '#simpay-10-date-6' ).val();
		endVal = $( '#simpay-10-date-7' ).val();

		if ( '' !== startVal && '' !== endVal ) {
			start = new Date( startVal );
			end = new Date( endVal );
			nights = end.getDate() - start.getDate();

			alert( 'Nights ' + nights );
			// Determine further pricing based on nights.
		}
	} );
	
} );