Time functionality snippets

time.cpp

#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#endif

#include <algorithm>
#include <chrono>
#include <ctime>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>

using namespace std;

inline time_t to_time_t(const std::chrono::system_clock::time_point& tp)
{
  // assume that a time_t is in seconds since 1/1/1970 UTC
  // and a system_clock::time_point has the same epoch
  return std::chrono::duration_cast<std::chrono::seconds>(
      tp.time_since_epoch()).count();
}

std::chrono::system_clock::duration get_tz_offset(bool include_dst = true)
{
  time_t now = time(nullptr);
  auto utc = std::gmtime(&now);
  time_t now_utc = mktime(utc);
  auto local = std::localtime(&now);
  time_t now_local = mktime(local);
  if (include_dst && local->tm_isdst) now_local += 60 * 60;

  auto offset = difftime(now_local, now_utc);
  return std::chrono::seconds(static_cast<int64_t>(offset));
}

inline std::chrono::system_clock::time_point to_local(
    const std::chrono::system_clock::time_point& tp)
{
  static auto tz_offset = get_tz_offset();
  return tp + tz_offset;
}

std::string to_iso_string(const std::chrono::system_clock::time_point& tp)
{
  auto t = to_time_t(tp);
  auto gmt = std::gmtime(&t);
  std::stringstream ss;
  ss << std::put_time(gmt, "%Y%m%dT%H%M%S.");
  auto tp_secs = std::chrono::system_clock::time_point{}
    + std::chrono::seconds(t);
  auto usecs = std::chrono::duration_cast<std::chrono::microseconds>(
      tp - tp_secs);
  ss << std::setw(6) << std::setfill('0') << usecs.count();
  return ss.str();
}

std::string to_log_string(const std::chrono::system_clock::time_point& tp)
{
  auto t = to_time_t(tp);
  auto gmt = std::gmtime(&t);
  std::stringstream ss;
  ss << std::put_time(gmt, "%Y-%m-%d %H:%M:%S.");
  auto tp_secs = std::chrono::system_clock::time_point{}
    + std::chrono::seconds(t);
  auto usecs = std::chrono::duration_cast<std::chrono::microseconds>(
      tp - tp_secs);
  ss << std::setw(6) << std::setfill('0') << usecs.count();
  return ss.str();
}

int main(void)
{
  auto now = std::chrono::system_clock::now();
  auto local_now = to_local(now);
  cout << to_log_string(now) << endl;
  cout << to_iso_string(local_now) << endl;
}