Remove post from latest posts after a month only with certain tag

This can be solved with hundreds of plugins, but I’m happy that someone wants to make it with code.

So that you want is actually not to hidden something but to just take the posts after the one month ago date.

$onemonth_before = date( 'Y-m-d',strtotime('-1 month') );
$args = array(
    //if you have custom post type 
    'post_type' => 'your-post-type',
    'posts_per_page' => 20,
    'post_status' => 'publish',
    'tag' => 'sometag',
    'date_query' => array(
        'after' => $onemonth_before,
        'inclusive' => true,
    ),
);
$query = new WP_Query( $args );

This maybe needs some changes to work in you case.

Some more info about the powerful date_query can be found in the
codex