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);
}