On Mon, Mar 16, 2026 at 11:52 PM Jonathan Wakely <[email protected]> wrote:
> This is a partial fix for PR 116110, so that zone changes that occur at
> a time specified as standard time will be parsed correctly. Previously
> all UNTIL times were assumed to be UTC, which is incorrect according to
> the spec. Only times with a 'u' suffix (or the equivalent 'g' or 'z')
> are UTC times. An 's' suffix means standard time (i.e. the zone's usual
> offset from UTC, without any DST adjustment) and a 'w' suffix (or no
> suffix) means wall time, so the offset and any DST adjustment. This
> commit fixes the handling of 's' suffixes, so that the ZoneInfo::m_until
> member is set correctly. It also fixes the handling of some UNTIL times
> using wall time, specifically those where the DST adjustment is known
> during parsing of the tzdata.zi file.
>
> This is not a complete fix, because as noted in PR 116110 a Zone line
> that refers to a named Rule and uses wall time cannot be determined
> while parsing tzdata.zi. We need to remember that the m_until member is
> not correct, and then adjust it later when we find the rule that applies
> at a given time point. That requires more work.
>
> Some existing tests need to be adjusted due to the fixes in this commit.
> The std/time/time_zone/get_info_sys.cc and std/time/zoned_time/1.cc
> tests are corrected to check that the changes happens at midnight local
> time, not midnight UTC as previously assumed.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/116110
> * src/c++20/tzdb.cc (operator>>(istream&, ZoneInfo&)): Adjust
> inf.m_until according to indicator suffix on AT time in UNTIL.
> * testsuite/std/time/time_zone/get_info_sys.cc: Adjust expected
> results to account for corrected logic.
> * testsuite/std/time/zoned_time/1.cc: Likewise.
> ---
>
> Tested x86_64-linux.
>
LGTM.
>
> libstdc++-v3/src/c++20/tzdb.cc | 15 +++++++++----
> .../std/time/time_zone/get_info_sys.cc | 22 +++++++++++--------
> .../testsuite/std/time/zoned_time/1.cc | 20 ++++++++++-------
> 3 files changed, 36 insertions(+), 21 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc
> b/libstdc++-v3/src/c++20/tzdb.cc
> index e26a9ee38072..82e4790b0d7f 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -2289,11 +2289,18 @@ namespace std::chrono
> at_time t{};
> // XXX DAY should support ON format, e.g. lastSun or Sun>=8
> in >> m >> d >> t;
> - // XXX UNTIL field should be interpreted
> - // "using the rules in effect just before the transition"
> - // so might need to store as year_month_day and hh_mm_ss and only
> - // convert to a sys_time once we know the offset in effect.
> inf.m_until = sys_days(year(y)/m.m/day(d)) + seconds(t.time);
> + if (t.indicator != at_time::Universal)
> + { // UNTIL uses "the rules in effect just before the
> transition"
> + // adjust by STDOFF
>
Was initially confused, if that shouldn't be previous offset, but this is
end time
(so the rule applies before transition) not start time.
> + inf.m_until -= seconds(inf.m_offset);
> + if (t.indicator != at_time::Standard)
> + {
> + if (inf.m_expanded) // Not a named Rule, SAVE is known
> now.
> + inf.m_until -= inf.m_save;
> + // else Named Rule, SAVE is unknown. FIXME: PR 116110
>
Yes, we will need to extract `find_letters` into a separate function, so we
can determine
save (DST offset) value for each transition.
> + }
> + }
> }
> else
> inf.m_until = sys_days(year::max()/December/31);
> diff --git a/libstdc++-v3/testsuite/std/time/time_zone/get_info_sys.cc
> b/libstdc++-v3/testsuite/std/time/time_zone/get_info_sys.cc
> index 769c7744017b..88f16b76ff48 100644
> --- a/libstdc++-v3/testsuite/std/time/time_zone/get_info_sys.cc
> +++ b/libstdc++-v3/testsuite/std/time/time_zone/get_info_sys.cc
> @@ -12,33 +12,37 @@ test_zurich()
> const time_zone* const tz = locate_zone("Europe/Zurich");
>
> {
> - sys_days d = 1853y/July/16;
> + sys_days d = 1853y/July/16; // On this date ...
> + auto offset = 34min + 8s; // ... local time is this far ahead of
> UTC,
> + auto t = d - offset; // so LMT to BMT transition is at this
> time.
>
> - auto info = tz->get_info(d - 1s);
> - VERIFY( info.offset == (34min + 8s) );
> + auto info = tz->get_info(t - 1s);
> + VERIFY( info.offset == offset );
> VERIFY( info.abbrev == "LMT" );
>
> - info = tz->get_info(d);
> + info = tz->get_info(t);
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
>
> - info = tz->get_info(d + 1s);
> + info = tz->get_info(t + 1s);
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
>
> - info = tz->get_info(d + 0.001s);
> + info = tz->get_info(t + 0.001s);
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
> }
>
> {
> sys_days d = 1894y/June/1;
> + auto offset = 29min + 46s;
> + auto t = d - offset;
>
> - auto info = tz->get_info(d - 1s);
> - VERIFY( info.offset == (29min + 46s) );
> + auto info = tz->get_info(t - 1s);
> + VERIFY( info.offset == offset );
> VERIFY( info.abbrev == "BMT" );
>
> - info = tz->get_info(d);
> + info = tz->get_info(t);
> VERIFY( info.offset == 1h );
> VERIFY( info.abbrev == "CET" );
> }
> diff --git a/libstdc++-v3/testsuite/std/time/zoned_time/1.cc
> b/libstdc++-v3/testsuite/std/time/zoned_time/1.cc
> index 1623aca1c7a8..a77cd99dd550 100644
> --- a/libstdc++-v3/testsuite/std/time/zoned_time/1.cc
> +++ b/libstdc++-v3/testsuite/std/time/zoned_time/1.cc
> @@ -48,24 +48,26 @@ test_zurich()
> const time_zone* const zurich = locate_zone("Europe/Zurich");
>
> {
> - sys_days d = 1853y/July/16;
> + sys_days d = 1853y/July/16; // On this date ...
> + auto offset = 34min + 8s; // ... local time is this far ahead of
> UTC,
> + auto t = d - offset; // so LMT to BMT transition is at this
> time.
>
> - auto z = zoned_seconds(zurich, sys_seconds(d) - 1s);
> + auto z = zoned_seconds(zurich, t - 1s);
> auto info = z.get_info();
> - VERIFY( info.offset == (34min + 8s) );
> + VERIFY( info.offset == offset );
> VERIFY( info.abbrev == "LMT" );
>
> - z = zoned_seconds(zurich, d);
> + z = zoned_seconds(zurich, t);
> info = z.get_info();
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
>
> - z = zoned_seconds(zurich, d + 1s);
> + z = zoned_seconds(zurich, t + 1s);
> info = z.get_info();
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
>
> - auto z2 = zoned_time(zurich, d + 0.001s);
> + auto z2 = zoned_time(zurich, t + 0.001s);
> info = z2.get_info();
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
> @@ -73,13 +75,15 @@ test_zurich()
>
> {
> sys_days d = 1894y/June/1;
> + auto offset = 29min + 46s;
> + auto t = d - offset;
>
> - auto z = zoned_seconds(zurich, sys_seconds(d) - 1s);
> + auto z = zoned_seconds(zurich, t - 1s);
> auto info = z.get_info();
> VERIFY( info.offset == (29min + 46s) );
> VERIFY( info.abbrev == "BMT" );
>
> - z = zoned_seconds(zurich, d);
> + z = zoned_seconds(zurich, t);
> info = z.get_info();
> VERIFY( info.offset == 1h );
> VERIFY( info.abbrev == "CET" );
> --
> 2.53.0
>
>
