How to get UTC offset (timezone) in correct format?

Question

I have code like this that converts an Epoch timestamp into GMT timestamp:

#include <array>
#include <ctime>
#include <string>

std::string getDateTimeZ(size_t epoch)
{
  const time_t time = epoch;
  const auto timeInfo = *localtime(&time);
  std::array<char, 80> buffer;
  std::strftime(buffer.data(), buffer.size(), "%Y-%m-%dT%H:%M:%S%z", &timeInfo);
  return buffer.data();
}

This works fine, except that my timestamp is e.g.:

2020-09-10T20:53:10+0300

I'd like it to be:

2020-09-10T20:53:10+03:00

How can I do this out-of-the-box without an ugly and error-prone hack on the resulting string? I don't see any other options to get the offset besides %z.

Some other library like Boost would also be acceptable.

Answer 1

You could use this preview of the C++20 <chrono> facilities, which works with C+11:

#include "date/tz.h"
#include <chrono>
#include <cstddef>
#include <string>
#include <sstream>

std::string
getDateTimeZ(std::size_t epoch)
{
    using namespace date;
    using namespace std::chrono;
    zoned_seconds zt{current_zone(), sys_seconds{seconds{epoch}}};
    std::ostringstream os;
    os << format("%FT%T%Ez", zt);
    return os.str();
}

The trick with this library is to use %Ez in place of %z.

This library does require some installation.