Formatting a std::chrono::system_clock renders in local time, not in UTC

[ned14]

For some time now we've had this in our cmake, so I figured time to upstream this:

  message(STATUS "NOTE: Patching '${VCPKG_INSTALLED_DIR}/${VCPKG_TARGET_TRIPLET}/include/fmt/chrono.h' to fix a formatting bug ...")
  file(READ "${VCPKG_INSTALLED_DIR}/${VCPKG_TARGET_TRIPLET}/include/fmt/chrono.h" fmt_chrono_contents)
  string(REPLACE "  auto format(std::chrono::time_point<std::chrono::system_clock> val," "  auto format(std::chrono::time_point<std::chrono::system_clock, Duration> val," fmt_chrono_contents "${fmt_chrono_contents}")
  string(REPLACE "    return formatter<std::tm, Char>::format(localtime(val), ctx);" "    return formatter<std::tm, Char>::format(gmtime(std::chrono::time_point_cast<std::chrono::system_clock::time_point::duration>(val)), ctx);" fmt_chrono_contents "${fmt_chrono_contents}")
  file(WRITE "${VCPKG_INSTALLED_DIR}/${VCPKG_TARGET_TRIPLET}/include/fmt/chrono.h" "${fmt_chrono_contents}")

Two things are changed here:

1. The Duration template parameter isn't passed through, breaking formatting time points with non-default duration types.

2. Formatting system clock time points is to UTC, not to localtime, as system clock specifically represents UTC not local time (if you want a local time point, use chrono::local_t). Microsoft's implementation of std::format agrees with this formatting interpretation i.e. system clocks format to UTC strings, local clocks format to local strings.

[vitaut]

The first change has already been applied. Could you by any chance submit the second one as a PR?

[ned14]

Last day of work for this year, so had a few spare minutes to get this done finally.

[ned14]

Apologies for the unprompted reply. Currently on holidays in the United States without any ability to write code, which is deliberate. Glad you figured out some way to merge it, will look into your remaining ask when I return to Europe in January.

[Jan-Otto-Bakkland-Volue]

Converting to local time not working as specified:
auto localTime = std::chrono::localtime(first_time)

or this:
auto localTime = std::localtime(first_time);

[vitaut]

@Jan-Otto-Bakkland-Volue, this doesn't look related to {fmt}. Please check cppreference or ask on StackOverflow.