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.