I just added the calendar_date_select date picker to the Rails app that I'm playing with. Right out of the box, it works quite nicely and even has formbuilder support! One of my favourite features is the ability to restrict the year range with just one option in the tag. Nice.
If this one doesn't work for you, you can still check out their page to see list of alternative date pickers. But for me, this one works just fine.