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

EDIT IN:

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