This is a knock-on of https://forum.literatureandlatte.com/t/lh4611-cant-enter-data-custom-metadata/50530/1
The challenge in that issue is that date custom fields are not clearly editable; not where to click or that it’s a right-click or even that a field exists.
But, once you do click… you still cannot use the keyboard to enter a date.
I have a custom date, YYYY-MM-DD format. (Casing is different for the actual format string.) In editing, 2002-06-01 will be morphed to 6/1/02. You cannot enter that field and replace the 6 with, well, anything. After a bunch of testing, I realized you must DELETE the value there first, because whatever you’re typing is inserted, not replacing, and if the value is not a valid date, it reverts.
The best fix is probably to change the cursor/mode to overwrite rather than insert for the date editor. Interestingly, using [Ins] to control overwrite defaults off, but even if enabled, it does not work in the Date editor. (That’d be another bug.)
Additionally, if you switch to another app, the date editor is closed.