"Date properties will only store the date, and must be set to midnight UTC for the date you want. "
My script was converting to my time zone, so subtracting 4 hours, and therefore breaking. SO I told my script that my timezone is actually GMT 0 (UTC) and any Deal property in my portal that was marked as a date picker type then started updating.
I can delete to post if you’d like, but we often leave solved topics on the forum to aid future posters with similar issues. Just let me know if you’d like me to delete this topic.