You could either do all the date generation on the client-side, or use Ajax to fetch date information from the server-side, which is what I did in my own date picker.
As far as where to go for one, I'm not sure. I wrote my own from scratch. I'd share that code with you, but its heavily dependent on my own PHP framework and wouldn't be usable outside of that. I'd help you write one, but that's a pretty complex project, and I just don't have that kind of time ;).
My advice is to Google around till you find something that fits your needs.
Good luck with your project!
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