Yesterday I've encountered the irritating date problem in Rails. Please remember to restart Webrick when you change the date conversion constants or time conversion constants with the 'merge!' method:
ActiveSupport::CoreExtensions::Date::Conversions::DATE_FORMATS.merge!( :default => "%d-%m-%Y" )
On reload it merges the array again... You can never remove the ':default' date with a merge...
*sigh*
This tip can save you a whole lot of time, because reloading doesn't work