A varchar named something like "DateAsEnscribed". With a related date field that you can search on with some well defined policy about what happens when gravestones have invalid dates enscribed them.

The problem is, you probably don't work out ou need that until you've got a million rows stored in a date column, and when you discover it, you then start asking ourself "I wonder how many of our dates have been auto-magically 'corrected' from accurate-but-invalid enscribed dates into valid-but-not-as-enscribed ones."