It's possible to get an exception when working with localised times

[musshorn]

Environment:
Windows 10 x64
MSVC Version 17.12.4

It;'s possible to pass a std::chrono::local_seconds object to format that has an ambiguous time, so for example ~2am on April 6th 2025 in Sydney daylight savings ends.

Sample Code (note, system running must be on AEDT)

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

int main()
{
    const auto& timeZoneDatabase = std::chrono::get_tzdb();
    const auto& currentZone = timeZoneDatabase.current_zone();
    std::cout << currentZone->name() << '\n'; // This needs to be Australia/Sydney

    uint64_t two_in_the_morning = 1743865205; // just after 2am on 2025/4/6 which is a daylight savings changeover point
    auto dur = std::chrono::duration<uint64_t>(two_in_the_morning);
    auto time_point = std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<uint64_t>>(dur);
    auto time_point_as_local_seconds = currentZone->to_local(std::chrono::floor<std::chrono::seconds>(time_point));

    std::string text = fmt::format("{:%Y-%m-%d %H:%M:%S}", time_point_as_local_seconds);
    std::cout << text << '\n';
    return 0;
}

This block here seems to be the cause:

fmt/include/fmt/chrono.h

Lines 575 to 580 in 94ab51c

	template <typename Duration,
	FMT_ENABLE_IF(detail::has_current_zone<Duration>())>
	inline auto localtime(std::chrono::local_time<Duration> time) -> std::tm {
	using namespace std::chrono;
	using namespace fmt_detail;
	return localtime(detail::to_time_t(current_zone()->to_sys<Duration>(time)));

to_sys can throw exceptions in these cases, and needs to be passed std::chrono::choose

[vitaut]

Could you submit a PR to fix this?

[vitaut]

BTW what do std::format and iostreams do in this case?

[musshorn]

Analyzing that is possibly a bit outside my ability, but I'll have a go:

std::format at least on MSVC seems check if the value is localized? Not 100% clear what that imbued stream does
https://github.com/microsoft/STL/blob/dfdccda510737c1e5fe8f84d7101df5aec269451/stl/inc/chrono#L5433

That gets passed to
https://github.com/microsoft/STL/blob/dfdccda510737c1e5fe8f84d7101df5aec269451/stl/inc/chrono#L5502

Which builds out the string.

[vitaut]

I meant does pick earliest or latest or throws an exception in your example?

[musshorn]

Oh it doesn't throw an exception, it simply prints:

Australia/Sydney
2025-04-06 02:00:05

I tried to dive in again, it doesn't seem to call to_sys at all. That would suggest my proposed pull is wrong.