Update post meta date always store 1970-01-07

With debugging enabled, you should have gotten the following message:

Notice: A non well formed numeric value encountered

This is because strtotime() expects an integer as its second argument. You could use it like so

$date = date('Y-m-d', strtotime('+1 week', strtotime($lejar_datum)));

or if you want to use the more modern approach with DateTime

$dt = DateTime::createFromFormat('Y-m-d', $lejar_datum)->modify('+1 week');
update_post_meta($job_id, '_job_expires', $dt->format('Y-m-d'));