code: 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)
  }
docs: // Time returns the timestamp part of the id.
  // It's a runtime error to call this method with an invalid id.