If you have created you custom post type you can add the metabox like this:
add_action( 'add_meta_boxes', 'add_dates' );
function add_dates() {
add_meta_box('date_select', 'Dates', 'date_select', 'customposttype', 'side', 'default');
}
function date_select() {
global $post;
echo '<input type="hidden" name="date_select_meta_noncename" id="date_select_meta_noncename" value="' .
wp_create_nonce( plugin_basename(__FILE__) ) . '" />';
$gender = get_post_meta($post->ID, '_date', true);
// Insert checkboxes HTML
}
Then you need to save the custom meta data.
function save_dates($post_id, $post) {
if ( !wp_verify_nonce( $_POST['date_select_meta_noncename'], plugin_basename(__FILE__) )) {
return $post->ID;
}
if ( !current_user_can( 'edit_post', $post->ID ))
return $post->ID;
$date_meta['_date'] = $_POST['_date'];
foreach ($date_meta as $key => $value) {
if( $post->post_type == 'revision' ) return;
$value = implode(',', (array)$value);
if(get_post_meta($post->ID, $key, FALSE)) {
update_post_meta($post->ID, $key, $value);
} else {
add_post_meta($post->ID, $key, $value);
}
if(!$value) delete_post_meta($post->ID, $key);
}
}
add_action('save_post', 'save_dates', 1, 2);
Of course you will need to set it up for you custom post type.
(Caveat: not tested)