The instructions on the website for filling out W-8BEN reads the following:
"Line 8: If you are providing this Form W-8BEN to
document yourself with respect to a financial account that
you hold with a U.S. office of a financial institution, provide
your date of birth. Use the following format to input your
information MM-DD-YYYY. For example, if you were born
on April 15, 1956, you would enter 04-15-1956."
So, I will put my date of birth in here. There are instructions on their website for explaining how to complete the Form but they are in American-speak and as clear as mud.