Godeps/_workspace/src/gopkg.in/mgo.v2/bson/bson.go
func (id ObjectId) Time() time.Time {
	// First 4 bytes of ObjectId is 32-bit big-endian seconds from epoch.
	secs := int64(binary.BigEndian.Uint32(id.byteSlice(0, 4)))
	return time.Unix(secs, 0)
}
func (id ObjectId) Time() time.Time {
	// First 4 bytes of ObjectId is 32-bit big-endian seconds from epoch.
	secs := int64(binary.BigEndian.Uint32(id.byteSlice(0, 4)))
	return time.Unix(secs, 0)
}
// Time returns the timestamp part of the id.
// It's a runtime error to call this method with an invalid id.