change the post time on multiple posts

If you have the id of the posts, you can use wp_update_post and change the post_date.

foreach ($postId as $id) {
  // random hour and time. You can also manually set these.
  $h = rand(1, 23);
  $i = rand(0, 59); 

  $newDate = array(
    'ID' => $id,
    'post_date' => "2013-12-19 $h:$i:59" // [ Y-m-d H:i:s ]
  );

  wp_update_post($newDate);
}