MongoMapper から created_at と updated_at の JSON 形式を返すと、次のようになります。
"updated_at\":\"2012-04-25T16:25:05Z\"
末尾のT(時間?)とZは何?
irb(main):009:0> Story.last.created_at
DEBUG - MONGODB (0ms) prototype_development['stories'].find({}).limit(-1)
=> 2012-04-25 16:24:26 UTC
irb(main):010:0> Story.last.created_at.to_s
DEBUG - MONGODB (0ms) prototype_development['stories'].find({}).limit(-1)
=> "2012-04-25 16:24:26 UTC"
irb(main):011:0> JSON.parse(Story.last.to_json)['created_at']
DEBUG - MONGODB (0ms) prototype_development['stories'].find({}).limit(-1)
=> "2012-04-25T16:24:26Z"