nanoDigits := [maxNanoSecondDigits]byte{'0', '0', '0', '0', '0', '0', '0', '0', '0'}
	for i := range len(sn) {
		switch c := sn[i]; {
		case c < '0' || c > '9':
			return time.Time{}, ErrHeader
		case i < len(nanoDigits):
			nanoDigits[i] = c
		}
	}
	nsecs, _ := strconv.ParseInt(string(nanoDigits[:]), 10, 64) // Must succeed after validation
	if len(ss) > 0 && ss[0] == '-' {