Object has no method 'joint'

The missing joint error

<> learnyounode run 10.js                                                                                         ["core_concepts/learnyou", "ron@yo"]
/home/ron/learn/node/nodeschool/core_concepts/learnyou/10.js:19
  var date = date.joint("-");
                  ^
TypeError: Object  has no method 'joint'
    at date_format (/home/ron/learn/node/nodeschool/core_concepts/learnyou/10.js:19:19)
    at Server.connect (/home/ron/learn/node/nodeschool/core_concepts/learnyou/10.js:8:11)
    at Server.EventEmitter.emit (events.js:95:17)
    at TCP.onconnection (net.js:1188:8)