Plugin to add metabox, field to select a date and a shortcode to use this value in my content

functions.php

<?php

/*
Plugin Name:  Metabox Date Select
Plugin URI:   https://example.com
Description:  Plugin to add metabox, field to select a date and a shortcode to use this value in my content.
Version:      1.0
Author:       Your Name
Author URI:   https://example.com
License:      GPL2
License URI:  https://www.gnu.org/licenses/gpl-2.0.html
Text Domain:  metabox-date-select
*/


// Register meta boxes
add_action( 'add_meta_boxes', 'metabox_date_select_register_meta_boxes' );

function metabox_date_select_register_meta_boxes() {
    add_meta_box( 'metabox_date_select_meta_box', __( 'Date Select', 'metabox-date-select' ), 'metabox_date_select_display_callback', 'post' );
}

// Display meta box
function metabox_date_select_display_callback( $post ) {
    wp_nonce_field( basename( __FILE__ ), 'metabox_date_select_nonce' );
    $date_value = get_post_meta( $post->ID, '_metabox_date_select', true );
    ?>
    <label for="metabox_date_select"><?php _e( 'Pick a date:', 'metabox-date-select' ); ?></label>
    <input type="date" name="metabox_date_select" id="metabox_date_select" value="<?php echo esc_attr( $date_value ); ?>" />
    <?php
}

// Save meta box
add_action( 'save_post', 'metabox_date_select_save_meta_box' );

function metabox_date_select_save_meta_box( $post_id ) {
    if ( ! isset( $_POST['metabox_date_select_nonce'] ) || ! wp_verify_nonce( $_POST['metabox_date_select_nonce'], basename( __FILE__ ) ) )
        return;

    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;

    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;

    if ( isset( $_REQUEST['metabox_date_select'] ) ) {
        update_post_meta( $post_id, '_metabox_date_select', sanitize_text_field( $_POST['metabox_date_select'] ) );
    }
}

// Shortcode
add_shortcode( 'metabox_date_select', 'metabox_date_select_shortcode' );

function metabox_date_select_shortcode( $atts ) {
    global $post;
    $date_value = get_post_meta( $post->ID, '_metabox_date_select', true );
    return $date_value;
}

?>