Is someone else wondering for what reason they have a need for your date of birth ?
There's a few knc devs on there so it's good to ask them a few questions. I made a thread there asking for a preview of the management website/etc and one of the knc guys uploaded a photo of him working on it.
You can also see the PCB there in the photo.
Just remember that not everyone uses bitcointalk.org forums. So for those people, they now have a place to hang out and discuss etc.
Birthday is pretty standard for pretty much any forum software. Even bitcointalk.org's. If you don't feel comfortable giving out your birthday then don't. Personally I didn't.
But what are you wondering about anyway? That they could be mining your details and stealing identity? lol