C++20でサマータイムを謳歌する

day_ight_saving.cpp

#include <iostream>
#include <date/date.h>
#include <date/tz.h>
#include <chrono>


void basic_calendar() {
	using namespace std::literals;
	using namespace date;

	constexpr year y{ 2018_y };
	constexpr month m{ aug };
	constexpr day d{ 9_d };
	constexpr year_month_day ymd{ 2018_y / aug / 9_d };
	static_assert(ymd == y / m / d);
	static_assert(2020_y / aug / 9_d != ymd);
	static_assert(2018_y <= 2020_y);
}

void basic_sys_or_local_time() {
	using namespace std::literals;
	using namespace date;

	constexpr auto hour_precision = sys_days{ 2018_y / aug / 9_d } +2h; // hour単位の精度。
	std::cout << hour_precision << "\n"; // 2018-08-09 0200
	constexpr auto second_precision = sys_days{ 2018_y / aug / 9_d } +2h + 2s; // second単位の精度。
	std::cout << second_precision << "\n"; // 2018-08-09 02:00:02
	constexpr auto hour_precision_local = local_days{ 2018_y / aug / 9_d } +2h;
	std::cout << hour_precision_local << "\n"; // 2018-08-09 0200
	constexpr auto second_precision_local = local_days{ 2018_y / aug / 9_d } +2h + 2s;
	std::cout << second_precision_local << "\n"; // 2018-08-09 02:00:02
	static_assert(hour_precision != second_precision);
	// static_assert(hour_precision != second_precision_local); コンパイル・エラー。
}

void basic_time_zone() {
	date::tzdb const& tz = date::get_tzdb(); // Timezone database
	date::time_zone const* const japan_time_zone = date::locate_zone("Asia/Tokyo"); // 日本の標準タイムゾーン。
}

void basic_zoned_time() {
	using namespace std::literals;
	using namespace date;

	auto z0 = make_zoned("Asia/Tokyo", sys_days{ 2018_y / aug / 9_d });   // Unix時刻の2018年8月9日00:00 == 日本時刻の2018年8月9日09:00
	auto z1 = make_zoned("Asia/Tokyo", local_days{ 2018_y / aug / 9_d }); // 日本時刻の2018年8月9日00:00。
	assert(z0 != z1);
	std::cout << z0 << "\n"; // 2018-08-09 09:00:00 JST
	std::cout << z1 << "\n"; // 2018-08-09 00:00:00 JST
}

void begining_day_light_saving_time() {
	using namespace std::literals;
	using namespace date;

	auto z = make_zoned("Asia/Tokyo", local_days{ 1948_y / may / 1 } +23h);

	for (int i = 0; i < 4; ++i) {
		std::cout << z << " = " << format("%F %T %Z\n", z.get_sys_time());
		z = z.get_local_time() + 1h;
	}
}

void begining_day_light_saving_time_with_sys_time() {
	using namespace std::literals;
	using namespace date;

	auto z = make_zoned("Asia/Tokyo", local_days{ 1948_y / may / 1 } +23h);

	for (int i = 0; i < 4; ++i) {
		std::cout << z << " = " << format("%F %T %Z\n", z.get_sys_time());
		z = z.get_sys_time() + 1h; // Unix時間を進める。
	}
}

void finishing_day_light_saving_time() {
	using namespace std::literals;
	using namespace date;

	auto z = make_zoned("Asia/Tokyo", local_days{ 1948_y / sep / sat[2] } +22h);

	for (int i = 0; i < 4; ++i) {
		std::cout << z << " = " << format("%F %T %Z\n", z.get_sys_time());
		z = z.get_local_time() + 1h;
	}
}

void finishing_day_light_saving_time_with_sys_time() {
	using namespace std::literals;
	using namespace date;

	auto z = make_zoned("Asia/Tokyo", local_days{ 1948_y / sep / sat[2] } +22h);

	for (int i = 0; i < 4; ++i) {
		std::cout << z << " = " << format("%F %T %Z\n", z.get_sys_time());
		z = z.get_sys_time() + 1h;
	}
}

void diff_across_day_light_saving() {
	using namespace std::literals;
	using namespace date;

	auto beforeJDT = make_zoned("Asia/Tokyo", local_days{ 1948_y / may / 1 } + 23h);
	auto afterJDT = make_zoned("Asia/Tokyo", local_days{ 1948_y / may / 2 }  + 1h);
	
	std::cout << date::format("%H:%M\n", afterJDT.get_sys_time() - beforeJDT.get_sys_time()); // 1時間差。
	std::cout << date::format("%H:%M\n", afterJDT.get_local_time() - beforeJDT.get_local_time()); // 2時間差。
}

int main()
try
{
	diff_across_day_light_saving();

	int x;
	std::cin >> x;
	return 0;
}
catch (date::nonexistent_local_time const& n) {
	std::cerr << n.what() << "\n";
}
catch (date::ambiguous_local_time const& a) {
	std::cerr << a.what() << "\n";
}