I always go for January 1st, 1970. It is (nearly) always accepted on the form because it is a perfectly valid date to be born, but I like to imagine that some analyst is pulling their hair out thinking they have a null coalescing bug.
My favorite irritating date is "1969-12-31" because now I know I have 2 problems: the coalescing bug and a timezone issue. :)
1970-01-01 is the way to go! The more, the merrier...
In Sweden in 1712 there was a February 30th. But never a 31st.