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The previous commit fixed most cases of %c formatting, but it incorrectly prints times using the system's local time zone. This only matters if the locale's %c format includes %Z, but some do. To print a correct value for %Z we can set tm.tm_zone to either "UTC" or the abbreviation passed to the formatter in the local-time-format-t structure. For local times with no info and for systems that don't support tm_zone (which is new in POSIX.1-2024) we just set tm_isdst = -1 so that no zone name is printed. In theory, a locale's %c format could use %z which should print a +hhmm offset from UTC. I'm unsure how to control that though. The new tm_gmtoff field in combination with tm_isdst != -1 seems like it should work, but using that without also setting tm_zone causes the system zone to be used for %Z again. That means local_time_format(lt, nullptr, &off) might work for a locale that uses %z but prints the wrong thing for %Z. This commit doesn't set tm_gmtoff even if _M_offset_sec is provided for a local-time-format-t value. libstdc++-v3/ChangeLog: PR libstdc++/117214 * configure.ac: Use AC_STRUCT_TIMEZONE. * config.h.in: Regenerate. * configure: Regenerate. * include/bits/chrono_io.h (__formatter_chrono::_M_c): Set tm_isdst and tm_zone. * testsuite/std/time/format/pr117214.cc: Check %c formatting of zoned_time and local time.