Remember to translate from the right Spanish sentence. The task of writing an English sentence correctly is just one of your responsibilities.
Not "...the date of your birth?"
"...your date of birth?"
Compare the following pair of sentences with the Duolingo exercise:
What is the date of your birth?
—¿Cuál es la fecha de tu nacimiento?
This is a common mistake. People get taught (incorrectly) that cuál=which and qué=what, but it's not so straightforward. Check this article for a better explanation... http://spanish.about.com/cs/grammar/a/que_vs_cual.htm