Sorry, I have picked the wrong category, please change it to frontend or what matches best.
And my third WP_Query initially was like this, but fetched empty values anyway:
$scheduled = array(
'numberposts' => -1,
'post_type' => 'post',
'order' => 'ASC',
'order_by' => 'meta_value',
'meta_key' => 'switchtime',
'meta_value' => '',
'meta_compare' => '!=',
'meta_type' => 'DATETIME'
);