It's definitely broken, but apparently by design. It's a deliberbug.
When you enter your birthday during Windows installation (yes, you have to have a Microsoft network account to log in to your own computer, and you have to supply a birthday), the button for confirming the date is a small, grey checkmark hiding below the date picker. Clicking the big, blue “Continue” button in the lower corner will discard your date choice (since it wasn’t picked) and then fail form validation because you didn’t pick a date.
There is a certain cubicle masochism associated with this kind of system. I’m convinced that computers did not get easier to use since all of them had command prompts. But you know, it’s got windows!