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我在mongodb中有这样的数据

{ 
    "latitude" : "", 
    "longitude" : "", 
    "course" : "", 
    "battery" : "0", 
    "imei" : "0", 
    "altitude" : "F:3.82V", 
    "mcc" : "07", 
    "mnc" : "007B", 
    "lac" : "2A83", 
    "_id" : ObjectId("4f0eb2c406ab6a9d4d000003"), 
    "createdAt" : ISODate("2012-01-12T20:15:31Z") 
}

我如何查询db.gpsdatas.find({'createdAt': ??what here??}),以便它从db返回上述数据结果？