"luckily my database was intact"
Perhaps not. Given that this works with other FMP database files, it is likely that either your layout or your database file was damaged.
Replace your file with a back up copy.
Make a new layout in your file from scratch (just add your date field to a blank layout and format it with the Calendar drop down.) and see if it works on this new layout. If so, then your layout was corrupted and you need to replace it.
Recover the file and test the recovered copy. If the recovered copy works correctly, replace it with an undamaged back up copy if at all possible.
Nuts. Making a new layout didn't work. What's strange is it works once and then once it corrupts the drop down calendar feature it won't work for any other layout or unrelated database until FMP is closed and restarted.
hmmmm.... looking for a good backup.
Thanks. Ran Recover on the database and although the records were "OK", it found errors, I pulled up an older version, cloned it and it was a snap to import the data from the recovered file. All is well.