Based on recent discussions in #2938, it can do a bit better with date cells that do not use a recognized date style. Improve differentiating `date` from `date+time`, and in deciding whether 4-digit years have been used in the input cells. This causes a change to one test, where the old result incorrectly expected a 2-digit year (you can verify by opening the spreadsheet in question that it is saved with a 4-digit year).