datepicker for custom post type admin

Issues I noticed in your code

• I don’t see where/how you’re calling that custom datepicker() function and you should use the same admin_enqueue_scripts hook for registering/enqueueing a stylesheet (.css) file.

• You should only load the CSS and JS files on the pages where the styles and scripts are being used, e.g. in your case, the post editing screen for your events post type, where the screen ID is events (the CPT slug).

• $post is not defined in your post_date_field() function — you should’ve used function post_date_field( $post ).

Making the Datepicker works

1. First, enqueue the CSS and JS files.

   Note: I used the Smoothness theme, but you can just choose whichever you like. I also used the jQuery UI Datepicker library that came bundled in WordPress core where the script handle name is jquery-ui-datepicker.

   function admin_styles() {
       if ( 'events' === get_current_screen()->id ) {
           wp_enqueue_style( 'jquery-ui-smoothness', // wrapped for brevity
               '//code.jquery.com/ui/1.12.1/themes/smoothness/jquery-ui.css', [], null );
       }
   }
   add_action( 'admin_enqueue_scripts', 'admin_styles' );
   
   function admin_scripts() {
       if ( 'events' === get_current_screen()->id ) {
           wp_enqueue_script( 'your-script', // wrapped for brevity
               '/path/to/your-script.js', [ 'jquery-ui-datepicker' ] );
       }
   }
   add_action( 'admin_enqueue_scripts', 'admin_scripts' );
   

2. Then, apply the Datepicker widget to your custom field.

   Note: I don’t use the custom datepicker() function. Instead, I called the Datepicker directly once the document is ready.

   // This code should be in your-script.js.
   
   jQuery( function ( $ ) {
       $( '#jquery-datepicker' ).datepicker();
   } );