Date fields are completely locked once you assign a person and move on to working elsewhere in your document. Once you assign someone to the date field and move on to working elsewhere, the field is completely locked
You can't change assignment, whether it's required, the position, or even delete it; you can do LITERALLY NOTHING with it, it's stuck there forever and the only thing you can do is delete the whole document and start all over again.
Tried this in the latest versions of both Firefox and Chrome.
Had to redo a document with nearly 200 field assignments because of this, so tedious.....