[ceptimus]

I don't use a separate flag. If the year is 1900 it means year not set. This is tough on people who are mostly now 123 years old, but at least they're more fortunate than those even older, where the database software won't even accept their birthday was a real date.

I suppose historians who often need to enter earlier dates in their records must use different database software, or split their dates across two or more fields rather than use the ISO format to fill a single field.