cpp_time.rst

Time

Table of Contents

• Timestamp
• To chrono::chrono::time_point
• Profiling
• Literals
• Format Time
• To time_t
• ISO 8601 format

Timestamp

// g++ -std=c++17 -Wall -Werror -O3 a.cc

#include <iostream>
#include <chrono>

using milliseconds = std::chrono::milliseconds;
namespace chrono = std::chrono;

int main(int argc, char *argv[])
{
  auto now = std::chrono::system_clock::now();
  auto t = now.time_since_epoch();
  std::cout << chrono::duration_cast<milliseconds>(t).count() << "\n";
}

To chrono::chrono::time_point

#include <iostream>
#include <iomanip>
#include <chrono>
#include <ctime>

namespace chrono = std::chrono;
using ms = std::chrono::milliseconds;

int main(int argc, char *argv[])
{
  using namespace std::literals;
  auto s = 1602207217323ms;
  chrono::system_clock::time_point tp(s);
  std::time_t t = chrono::system_clock::to_time_t(tp);
  std::cout << std::put_time(std::gmtime(&t), "%FT%TZ") << "\n";
}

Profiling

#include <iostream>
#include <chrono>

#include <unistd.h>

using milliseconds = std::chrono::milliseconds;
namespace chrono = std::chrono;

int main(int argc, char *argv[])
{
  auto start = std::chrono::steady_clock::now();
  sleep(3);
  auto end = std::chrono::steady_clock::now();
  auto d = end - start;
  std::cout << chrono::duration_cast<milliseconds>(d).count() << "\n";
}

Literals

#include <iostream>
#include <chrono>

using ms = std::chrono::milliseconds;
namespace chrono = std::chrono;

int main(int argc, char *argv[])
{
  using namespace std::literals;
  auto t = 1602207217323ms;
  std::cout << std::chrono::duration_cast<ms>(t).count() << "\n";
}

Format Time

#include <iostream>
#include <iomanip>
#include <ctime>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  std::time_t t = std::time(nullptr);
  constexpr char fmt[] = "%c %Z";
  std::cout << "UTC " << std::put_time(std::gmtime(&t), fmt) << "\n";
  std::cout << "Local " << std::put_time(std::localtime(&t), fmt) << "\n";

  std::string tz = "America/Chicago";
  putenv(tz.data());
  std::cout << "Chicago " << std::put_time(std::localtime(&t), fmt) << "\n";
}

To time_t

#include <iostream>
#include <iomanip>
#include <chrono>
#include <ctime>

namespace chrono = std::chrono;

int main(int argc, char *argv[])
{
  auto now = chrono::system_clock::now();
  std::time_t t = std::chrono::system_clock::to_time_t(now);
  std::cout << std::put_time(std::gmtime(&t), "%FT%TZ") << "\n";
}

ISO 8601 format

#include <iostream>
#include <iomanip>
#include <chrono>
#include <ctime>

namespace chrono = std::chrono;

int main(int argc, char *argv[])
{
  auto now = chrono::system_clock::now();
  std::time_t t = std::chrono::system_clock::to_time_t(now);
  std::cout << std::put_time(std::gmtime(&t), "%Y-%m-%dT%H:%M:%SZ") << "\n";
  std::cout << std::put_time(std::gmtime(&t), "%FT%TZ") << "\n";
  std::cout << std::put_time(std::gmtime(&t), "%FT%TZ%z") << "\n";
}