Format a date like 1st, 2nd, 3rd

Source: [Helper to print numbers as ordinals (1st, 2nd, 3rd...)](http://snippets.dzone.com/posts/show/593)

    class Fixnum
      def ordinal
        self.to_s + ( (10...20).include?(self) ? 'th' : %w{ th st nd rd th th th th th th }[self % 10] )
      end
    end

    d = Time.now.day
    puts 'today is the ' + d.ordinal
    #=> today is the 10th

date format ordinals