Interesting...

irb(main):008:0> Date.today.freeze.to_s

TypeError: can't modify frozen object
        from (eval):4:in `jd'
        from /usr/local/lib/ruby/1.8/date.rb:769:in `__16745__'
        from (eval):4:in `civil'
        from /usr/local/lib/ruby/1.8/date.rb:781:in `year'
        from /usr/local/lib/ruby/1.8/date/format.rb:502:in `strftime'
        from /usr/local/lib/ruby/1.8/date/format.rb:456:in `scan'
        from /usr/local/lib/ruby/1.8/date/format.rb:456:in `strftime'
        from /usr/local/lib/ruby/1.8/date/format.rb:468:in `strftime'
        from /usr/local/lib/ruby/1.8/date/format.rb:456:in `scan'
        from /usr/local/lib/ruby/1.8/date/format.rb:456:in `strftime'
        from /usr/local/lib/ruby/1.8/date.rb:1022:in `to_s'
        from (irb):8


Gavin (ruby 1.8.1 (2003-12-20) [i386-cygwin])