I just ran into an odd issue. I’m trying to fix posts that were once published and then removed and others that were scheduled but missed the scheduled post date for one reason or another.
What would happen is each time I click Reset then save the post date would revert back to the old date.
I found 2 fixes for this.
First, if I changed the date to a future date, schedule the post, and then reset it I could get it to go back to a draft as I wanted.

Second, I could disable Edit Flow (Link to WordPress.org: Edit Flow) and this also fixed the issue. But I really like Edit Flow and use it to help manage content and writers.