Sortable column with custom data by date

Question

I would like to sortable a column with custom data. I have a custom post type named "events" and I have a custom meta_key named "_start_at".

My column is sortable but when I click on, my posts are ordrered by date of publication and not by date "start_at".

How can i do this ?

This is my code :

function my_set_sortable_columns( $columns )
{
    $columns['dateevent'] = 'dateevent';
    return $columns;
}
add_filter( 'manage_edit-event_sortable_columns', 'my_set_sortable_columns' );

function my_sort_custom_column_query( $query )
{
    $orderby = $query->get( 'orderby' );

    if ( 'dateevent' == $orderby ) {

        $meta_query = array(
            'relation' => 'OR',
            array(
                'key' => '_start_at',
                'compare' => 'NOT EXISTS',
            ),
            array(
                'key' => '_start_at',
            ),
        );
        $query->set( 'meta_query', $meta_query );
        $query->set( 'orderby', '_start_at' );
    }
}
add_action( 'pre_get_posts', 'my_sort_custom_column_query' );

Here it's an example of data for the _start_at meta value :

Answer 1

Thanks a lot to @Sally CJ for the answers. It works with changing format of start_at field and adding key in array like this :

function my_sort_custom_column_query( $query )
{
    $orderby = $query->get( 'orderby' );

    if ( 'dateevent' == $orderby ) {

        $meta_query = array(
            'relation' => 'OR',
            '_start_at' => array(
                'key' => '_start_at',
                'type' => 'DATETIME',
            ),
        );
        $query->set( 'meta_query', $meta_query );
        $query->set( 'orderby', '_start_at' );
    }
}
add_action( 'pre_get_posts', 'my_sort_custom_column_query' );