You would like to start your rails console in an environment that is not the development, like staging or production.
Using the latest 3 shortcut you can use:
rails c staging
You have the error showing up after starting either the server (./script/server) or the console (./script/console).
For my particular case it started appearing after installing the Rails 3.0 beta3 version, and in an application developed with Rails 2.3.2.
Just install the system_timer gem:
sudo gem install system_timer