Looks the same every time. Tried to deactivate all plugins, used <?php the_date('Y-m-d') ?>. It stays like that. Also the problem appears on new created posts. Not on all of them though.. Some days ago the date was missing on one post again.
Now i am using <?php wp_days_ago_v3 ($stopUsingAjaxAfter, $showDateAfter, $showDateFormat, $showYesterday) ?> because using WP-days-ago. now i have the date on all posts.