ObjectId

An ObjectID class is available to scripts in order to facilitate working with BSON object ids used to represent unique identifiers.

ObjectId(string/date/number) Takes a hex string representation of an object id, a date or a timestamp in milliseconds.

Run this script

return {
    date: new ObjectID(new Date()),
    dateDate: new ObjectID(new Date()).toDate(),
    id: new ObjectID(),
    idDate: new ObjectID().toDate(),
    str: new ObjectID("4a232bca18759fcbdef24855"),
    strDate: new ObjectID("4a232bca18759fcbdef24855").toDate(),
    ts: new ObjectID("4a232bca18759fcbdef24855").getTimestamp()
}

And you might get this

{
    "date": "56d615040000000000000000",
    "dateDate": "2016-03-01T22:17:40.000Z",
    "id": "56d615048cacf5d0013e9305",
    "idDate": "2016-03-01T22:17:40.000Z",
    "str": "4a232bca18759fcbdef24855",
    "strDate": "2009-06-01T01:15:54.000Z",
    "ts": 1243818954
}

id.toString() returns the BSON id as a hexstring

id.toJSON() Used primarily by JSON.stringify(), returns the BSON id as a hexstring.

id.equals(other) Returns true if the passed in argument contains the same value as the id. The argument may be a hexstring or an ObjectID instance.

id.toDate() Returns the time the ObjectID was create as a Date value.

id.getTimestamp() Returns the number of seconds between midnight of January 1, 1970 and the time the ObjectID was created.