NTP Timestamp Format(64bit) to ISO 8601(extended format) + fraction

fmt_ntpts.cpp

std::string fmt_ntpts(uint64_t ts)
{
	// 1970/1/1(UNIT time epoch) - 1900/1/1(NTP timestamp epoch)
	const uint32_t offset = (24u * 60 * 60) * 25567;
	time_t t = (ts >> 32) - offset;
	double d = (ts & 0xffffffff) / (double)(1ULL << 32);
	char buf[256];
	size_t n = std::strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%SZ", std::gmtime(&t));
	std::sprintf(buf + n, "%f", d);
	buf[n] = ' ';	// force erase '0'
	return buf;
}