prefix_update_time_field.php

<?php
/**
 * JS function to dynamically remove options from an add-ons select field
 * Update the field ID of your add-ons select field in line 12 below
 */
function prefix_update_time_field() {
	?>
	<script>
		jQuery( document ).ready( function( $ ) {

		  // This is the ID of the select field listing the possible times
		  var field_id = 911;
		  
		  var field = $( '.pewc-field-' + field_id ).find( 'select' );
		  var first_option = $( field ).find( "option:first-child" ).text();
		  
		  // This is a list of the available times per day
		  function get_times( day ) {
			var times = [
			  [ '10:00' ], // Sunday
			  [ '10:00', '14:00', '17:00' ], // Monday
			  [ '17:00' ], // Tuesday
			  [ '10:00', '14:00', '17:00' ], // Wednesday
			  [ '17:00' ], // Thursday
			  [ '10:00', '14:00', '17:00' ], // Friday
			  [ '10:00' ] // Saturday
			];
			return times[day];
		  }

		  $( ".bfws-start-date" ).on( 'change', function() {
			
			var date = $(this).datepicker('getDate');
			var day_of_week = date.getUTCDay();
			var times = get_times( day_of_week );
			
			// Remove all options from time field except first option
			$( field ).empty();
			$( field ).append( '<option value="">' + first_option + '</option>' );
			
			// Repopulate with new options
			for( var i = 0; i < times.length; i++ ) {
			  $( field ).append( '<option value="' + times[i] + '">' + times[i] + '</option>' );
			 }
			
		  });
		  
		});
	</script>
	<?php
}
add_action( 'wp_footer', 'prefix_update_time_field' );