Originally from ticket #1211.
By default you can't have event date in those bubbles. BUT, I modified a little bit theme's code. Attached is a file, it should stay in /lib/php/ directory -> post.class.php . I modified the static function list_view() so it would show start date for events, in timeline (lines 116-135 are added by me, so keep this file safe on updates). If something goes wrong, you can switch any time to the default file.
Have a good day!