zk_republish_block_to
$meta_query = array( 'relation' => 'OR', array( 'key' => 'zk_republish_block_to', 'compare' => 'NOT EXISTS', ), array( 'key' => 'zk_republish_block_to', 'value' => '$date', 'compare' => '<', ), );
$meta_query = array( 'relation' => 'OR', array( 'key' => 'zk_republish_block_to', 'compare' => 'NOT EXISTS', ), array( 'key' => 'zk_republish_block_to', 'value' => '$date', 'compare' => '<', ), );