Scheduled posts are published with the modified_date of when you wrote the post and the post_date of when it was published.
For a non-scheduled post the modified_date and post_date are the same when the post is published.

To see the Code click here.To hide the Code click here.

To get an idea of what your post is using, especially scheduled posts just drop this bit of code temporarily into “the_Loop

global $post;
echo 'Post Date: ' . $post->post_date;
echo '<br />';
echo 'Post Modified (date): ' . $post->post_modified;