Thank you to everyone who replied. I've found my answer, and although it was
more complicated than I thought, and also required some "tweaking" -- it
works the way I want it to.
You can find the key piece on the web he
http://addbalance.com/word/datefields2.htm
This guy has a field that was created by a macro that is complicated to say
the least. I did not bother with the macro, but instead downloaded his
sample document with the field, copy and pasted it into my target document --
and then edited the field to make it work.