When working on a draft one cannot change the date of the post before the post is published.
Steps to reproduce include:
Make a new post
set as draft
Click Advanced Editing
Check Edit Timestamp
Change date, click Save and Continue Editing
Notice that the date was not modified. Saving, and then going back to the draft also does not work. Editing the timestamp will not work until a post is published.
If this behavior is intentional, albeit misleading, then the expected behavior is desirable in instances such as where one is taking a personal site and switching it to run on WordPress. For instance, my site is something I was originally doing in plain HTML using vim, so I've had to import my posts by hand.