time.cpp

// See https://searchfox.org/mozilla-central/source/js/src/vm/DateTime.cpp

#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <time.h>

#include <iostream>

#if !defined(XP_WIN)
#  include <limits.h>
#  include <unistd.h>
#endif /* !defined(XP_WIN) */

#define HAVE_LOCALTIME_R 1

static bool ComputeLocalTime(time_t local, struct tm* ptm) {
#if defined(_WIN32)
  return localtime_s(ptm, &local) == 0;
#elif defined(HAVE_LOCALTIME_R)
  return localtime_r(&local, ptm);
#else
  struct tm* otm = localtime(&local);
  if (!otm) {
    return false;
  }
  *ptm = *otm;
  return true;
#endif
}

static bool ComputeUTCTime(time_t t, struct tm* ptm) {
#if defined(_WIN32)
  return gmtime_s(ptm, &t) == 0;
#elif defined(HAVE_GMTIME_R)
  return gmtime_r(&t, ptm);
#else
  struct tm* otm = gmtime(&t);
  if (!otm) {
    return false;
  }
  *ptm = *otm;
  return true;
#endif
}

static int32_t UTCToLocalStandardOffsetSeconds() {
  constexpr double SecondsPerMinute = 60;
  constexpr unsigned SecondsPerHour = 60 * 60;
  constexpr unsigned SecondsPerDay = SecondsPerHour * 24;

  time_t currentMaybeWithDST = time(nullptr);
  if (currentMaybeWithDST == time_t(-1)) {
    return 0;
  }

  struct tm local;
  if (!ComputeLocalTime(currentMaybeWithDST, &local)) {
    return 0;
  }

  time_t currentNoDST;
  if (local.tm_isdst == 0) {
    currentNoDST = currentMaybeWithDST;
  } else {
    struct tm localNoDST = local;
    localNoDST.tm_isdst = 0;

    currentNoDST = mktime(&localNoDST);
    if (currentNoDST == time_t(-1)) {
      return 0;
    }
  }

  struct tm utc;
  if (!ComputeUTCTime(currentNoDST, &utc)) {
    return 0;
  }

  int utc_secs = utc.tm_hour * SecondsPerHour + utc.tm_min * SecondsPerMinute;
  int local_secs = local.tm_hour * SecondsPerHour + local.tm_min * SecondsPerMinute;
  if (utc.tm_mday == local.tm_mday) {
    return local_secs - utc_secs;
  }
  if (utc_secs > local_secs) {
    return (SecondsPerDay + local_secs) - utc_secs;
  }
  return local_secs - (utc_secs + SecondsPerDay);
}

int main() {
  do  {
   std::cout << "current time zone offset: " << UTCToLocalStandardOffsetSeconds() << std::endl;
  } while (std::cin.get() == '\n');
  return 0;
}