code: func Timestamp(t time.Time) uint64 {
  	return uint64(t.Unix())*1000 +
  		uint64(t.Nanosecond()/int(time.Millisecond))
  }
docs: // Timestamp converts a time.Time to Unix milliseconds.
  //
  // Because of the way ULID stores time, times from the year
  // 10889 produces undefined results.