Hi David - I don't think it is related to the Windows bug. I used the TDateEdit in 10.1.2 Berlin and it worked fine. Used it in Tokyo and it does not.
After installing the hotfix, the TDateEdit does not change the displayed month (see https://quality.embarcadero.com/browse/RSP-18348). Is there a way to rollback the April hotfix (without having to go thru a full re-install of Tokyo)?