The url suggester for the blog does not know about the post date, it only knows the title of the post and the current date so it cannot suggest url dates correctly for past dates. You would have to manually edit the suggested url to set a past date.
It does what it was designed for which is for new blog posts to make it easy to suggest an url with the current date. To try and make it support dates in the past would add a lot of complexity that I do not want to add.
Best,
Joe