change this line:

 $post_date = human_time_diff(get_the_time('U', $post_id), current_time('timestamp')) . ' ' . __('ago', 'cosmotheme'); 

with this one:

 $post_date =  __('ago', 'cosmotheme') . ' ' . human_time_diff(get_the_time('U', $post_id), current_time('timestamp'));