YYYY MM DD is the format, no dashes, or slashes (as indicated by DFS website) Took me  near a half hour to figure it out, hope this helps somebody out there! 
I can't believe Dell is really this bad of a website. I have been trying to create my Dell Financial Services account. But when validating the account, it keeps saying my date of birth is wrong. I have tried every which way to enter my date of birth in different formats and it keeps saying invalid date. This is so infuriating. And this stupid thing they haven't fixed in a decade. Shame on you guys.
Dell Financial Birth Date Format? #2