A simple and clean way (and also the preffered way of doing this) is by a Rails plugin.
Create a folder under vendor/plugins, maybe "date_extensions", create an "init.rb" file. Your init.rb would look like this:
Date.class_eval do
def is_older_than( other_date ) self < other_date end
end
And so on, adding the other methods you want. For more about ruby method definition, read the metaprogramming chapter from -> http://oreilly.com/catalog/9780596516178/toc.html