Declare timestamps! in your model to add created_at and updated_at keys to your document. The values will be automatically set whenever the document is saved.
key :name, String
u = User.create(:name => 'John')
u.created_at # => Sat, 18 Dec 2010 14:52:55 UTC +00:00
u.updated_at # => Sat, 18 Dec 2010 14:52:55 UTC +00:00
u.name = 'Brandon'
u.updated_at # => Sat, 18 Dec 2010 14:53:07 UTC +00:00