Godeps/_workspace/src/gopkg.in/mgo.v2/bson/bson.go
func NewObjectIdWithTime(t time.Time) ObjectId {
	var b [12]byte
	binary.BigEndian.PutUint32(b[:4], uint32(t.Unix()))
	return ObjectId(string(b[:]))
}
func NewObjectIdWithTime(t time.Time) ObjectId {
	var b [12]byte
	binary.BigEndian.PutUint32(b[:4], uint32(t.Unix()))
	return ObjectId(string(b[:]))
}
// NewObjectIdWithTime returns a dummy ObjectId with the timestamp part filled
// with the provided number of seconds from epoch UTC, and all other parts
// filled with zeroes. It's not safe to insert a document with an id generated
// by this method, it is useful only for queries to find documents with ids
// generated before or after the specified timestamp.