Formatting std::chrono::time_point with double Rep or before Epoch

[zeroxia]

Hi,

I have two questions concerning formatting the time_point object with fmt:

1. It seems that it cannot format time_point with Rep set to double, could this be improved?
2. It seems that it incorrectly formats timepoint before Epoch, you can see negative sub-seconds number in the output.

Compiler Explorer: https://godbolt.org/z/qd4Y3b7P9

Code:

#include <fmt/format.h>
#include <fmt/chrono.h>
#include <fmt/ostream.h>

template <typename Dur>
std::string getTimestamp(const std::chrono::time_point<std::chrono::system_clock, Dur>& tp)
{
    using namespace std::chrono;
    const auto tp_ms = time_point_cast<milliseconds>(tp);
    const auto milli_count = (tp_ms.time_since_epoch()).count() % 1000;
    return fmt::format("{:%Y-%m-%d_%H:%M:%S}.{:03d}", tp_ms, milli_count);
}

template <typename Dur>
std::string getTimestamp2(const std::chrono::time_point<std::chrono::system_clock, Dur>& tp)
{
    using namespace std::chrono;
    const auto milli_count = duration_cast<milliseconds>(tp.time_since_epoch()).count() % 1000;
    return fmt::format("{:%Y-%m-%d_%H:%M:%S}.{:03d}", tp, milli_count); // Passing tp to format directly
}

std::chrono::system_clock::time_point makeTimePoint(const std::tm& tm1, double fraction)
{
    struct std::tm tm2 = tm1;
    const auto t = std::mktime(&tm2);

    using namespace std::chrono;
    constexpr auto DEN = system_clock::duration::period::den;
    const auto count_subsecs = static_cast<system_clock::rep>(t) * DEN +
                               static_cast<system_clock::rep>(fraction * DEN);
    system_clock::time_point tp(system_clock::duration{count_subsecs});
    return tp;
}

int main()
{
    const auto tp = std::chrono::system_clock::now();
    const auto tp2 = std::chrono::time_point_cast<std::chrono::duration<double>>(tp);

    fmt::print("timepoint: {}\n", getTimestamp(tp));
    fmt::print("timepoint: {}\n", getTimestamp(tp2));

    // Issue 1:
    // fmt::print("timepoint: {}\n", getTimestamp2(tp2)); // This causes build error at line 19 above.

    // Issue 2:
    std::tm tm{};
    tm.tm_year = 1969 - 1900;
    tm.tm_mon = 12 - 1;
    tm.tm_mday = 31;
    tm.tm_hour = 23;
    tm.tm_min = 59;
    tm.tm_sec = 59;
    const auto tp3 = makeTimePoint(tm, 0.123456);
    fmt::print("timepoint: {}\n", getTimestamp(tp3)); // timepoint: 1970-01-01_00:00:00.-876
}

[vitaut]

It seems that it cannot format time_point with Rep set to double, could this be improved?

Yes, this can be done by adding Duration template argument in

fmt/include/fmt/chrono.h

Line 2020 in ad91cab

auto format(std::chrono::time_point<std::chrono::system_clock> val,

A PR is welcome.

It seems that it incorrectly formats timepoint before Epoch, you can see negative sub-seconds number in the output.

I am actually not sure what should happen in this case. Does the standard say anything about it?

[ShawnZhong]

The first issue is resolved by #3115. but the second one remains:

#include <fmt/chrono.h>
#include <fmt/core.h>

#include <chrono>

int main() {
  {
    auto tp = std::chrono::system_clock::from_time_t(0) +
              std::chrono::milliseconds(250);
    fmt::print("{:%Y-%m-%d %H:%M:%S}\n", tp); // 1970-01-01 00:00:00.250000
  }

  {
    auto tp = std::chrono::system_clock::from_time_t(0) -
              std::chrono::milliseconds(250);
    fmt::print("{:%Y-%m-%d %H:%M:%S}\n", tp); // 1970-01-01 00:00:00.250000
  }
}