add-field-on-checkout.php

<?php

add_action( 'woocommerce_after_checkout_billing_form', 'display_extra_fields_after_billing_address' , 10, 1 );

function display_extra_fields_after_billing_address () {
	_e( "Delivery Date: ", "add_extra_fields");
	?>
	<br>
	<input type="text" name="add_delivery_date" class="add_delivery_date" placeholder="Delivery Date">

	<script>
	    jQuery(document).ready(function( $ ) {
	        $( ".add_delivery_date").datepicker( {
	        	minDate: 0,	
	        } );
	     } );
	</script>
	<?php 
}

add_action( 'wp_enqueue_scripts', 'enqueue_datepicker' );

function enqueue_datepicker() {
    if ( is_checkout() ) {
        // Load the datepicker script (pre-registered in WordPress).
         wp_enqueue_script( 'jquery-ui-datepicker' );

         // You need styling for the datepicker. For simplicity I've linked to Google's hosted jQuery UI CSS.
         wp_register_style( 'jquery-ui', '//code.jquery.com/ui/1.11.2/themes/smoothness/jquery-ui.css' );
         wp_enqueue_style( 'jquery-ui' );  
    }  
}