strftime.go
func formatNano(nanosec uint, n int, trim bool) []byte {
	u := nanosec
	var buf [9]byte
	for start := len(buf); start > 0; {
		start--
		buf[start] = byte(u%10 + '0')
		u /= 10
	}

	if n > 9 {
		n = 9
	}
	if trim {
		for n > 0 && buf[n-1] == '0' {
			n--
		}
		if n == 0 {
			return buf[:0]
		}
	}
	return buf[:n]
}
func formatNano(nanosec uint, n int, trim bool) []byte {
	u := nanosec
	var buf [9]byte
	for start := len(buf); start > 0; {
		start--
		buf[start] = byte(u%10 + '0')
		u /= 10
	}

	if n > 9 {
		n = 9
	}
	if trim {
		for n > 0 && buf[n-1] == '0' {
			n--
		}
		if n == 0 {
			return buf[:0]
		}
	}
	return buf[:n]
}
// A load from pkg/time/format.go of golang source code.
// formatNano appends a fractional second, as nanoseconds, to b
// and returns the result.