Unable to save datetime custom meta field using update_post_meta() function

Question

When I do something like this (in my plugin function inside WP admin):

$metaboxes = get_post_meta( $post_id );
echo "<pre>";
print_r($metaboxes);
echo "</pre>";
die();

I get all my metaboxes info except the datetime.

Why is that? Where are they stored and how can I access them?

I am using this plugin http://www.deluxeblogtips.com/meta-box/ to add custom meta fields to my content type like:

array(
  'name'  => 'Date and time:',
  'type'  => 'datetime',
  'id'    => "rw_datetime",
),

I want to save the datetime field if some other field has certain value using save_post function and this is not working for datetime for some reason:

update_post_meta($post_id, 'rw_datetime', '2013-08-28 11:11' );

But something like this for a text field is working nice:

update_post_meta($post_id, 'rw_some_textfield', 'This is working!' );

Why is that? Why it's not working for datetime and why it is working for other field types?

How can I grab the datetime value if I know the $post_id and then update this field using update_post_meta() function?

UPDATE:

Code of my save_post function:

add_action( 'save_post', 'things_before_save' ); 

function things_before_save( $post_id ) { 

    // No auto saves 
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return; 

    // make sure the current user can edit the post 
    if( ! current_user_can( 'edit_post' ) ) return;

    // assure the post type
    if ( ! get_post_type($post_id) == 'cards' ) return;

    // working
    update_post_meta($post_id, 'rw_some_textfield', 'This is working!' );

    // not working ;(
    update_post_meta($post_id, 'rw_datetime', '2013-08-28 11:11' );

} 

Notice that I am not working with any variables, just with strings This is working! and 2013-08-28 11:11 to eliminate possible errors.

So, I guess the problem is that rw_datetime is stored somewhere else and not at the same place as other metaboxes.

Answer 1

it seems that you're using that plugin in a wrong way. It's not necessary hook to save_post because the plugin do it for you. Simply register your meta box and your fields. To retrieve the values check this