datejs-rails wraps the datejs library in a rails engine for simple use with the asset pipeline provided by rails 3.1. The gem includes the last official release of datejs, which is from 2007.
Add the following to your gemfile:
//= require date
This will require the en-US version of datejs. All other localizations are available as well. For instance, you could instead:
//= require date-fr-FR
//= require datejs-extras