在mongodb中如何存入日期？

ubuntu@ip-10-0-1-223:~$ mongo 10.0.1.223

MongoDB shell version: 2.4.9

connecting to: 10.0.1.223/test

> db.penguins.insert({"penguin": "skipper"})

> db.penguins.insert({"penguin": "kowalski"})

>

> use penguins

switched to db penguins

> db.penguins.find()

{ "_id" : ObjectId("5498da1bf83a61f58ef6c6d5"), "penguin" : "skipper" }

{ "_id" : ObjectId("5498da28f83a61f58ef6c6d6"), "penguin" : "kowalski" }

> db.penguins.find().forEach(function (doc){ d = doc._id.getTimestamp(); print(d.getFullYear()+"-"+(d.getMonth()+1)+"-"+d.getDate() + " " + d.getHours() + ":" + d.getMinutes() + ":" + d.getSeconds()) })

2014-12-23 3:4:41

2014-12-23 3:4:53