Extending core classes?

I want to add some functionality to DateTime and stuff. I'm not sure how I would go about extending it though...would I modify the class itself in the core, extend it in another class, etc? Also, I don't see where I'd put it in my rails application....thanks!

Ryan's right -- Ruby classes are always open for extension. As for where to put it in a Rails app... the 'lib' folder is probably the most logical place if you're working on code that is specific to your application. If it's something that you'd like to share with the world (or possibly just some of your other projects) then build it into a plugin.