I just registered in ServerFault but my user page said that im member for 24 days.
is it a known bug ?
I think that just means that you visited the site 24 days before for the first time.
Accounts are very lightweight as Jeff mentioned in one of his blog posts and you get one when you just visit. When you register details are added to your already existing account.
From the blog post:
Stack Overflow is a bit unusual in that our accounts are ultra lightweight. We don’t require registration; we allow essentially anonymous posting. We were inspired by Wikipedia in this regard. We wanted to reduce the friction of asking and answering to little more than entering an anonymous comment on a blog. For example, we don’t always even have an email address for our users. And we’re fine with that. I can’t think of very many sites with as loose a definition of account as Stack Overflow
Technically there was a cookie set in your browser 24 days before you registered.