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Fix time.mktime() for zones whose standard UTC offset changes mid-year #2096

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Merged
slozier merged 6 commits into from
Oct 11, 2026
Merged

Fix time.mktime() for zones whose standard UTC offset changes mid-year #2096

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1ae8460
Fix time.mktime() for zones whose standard UTC offset changes mid-year
BCSharp Oct 9, 2026
69dfaf7
Extend the implementation to .NET Framework
BCSharp Oct 9, 2026
4a7251f
Add test for RemoveDst
BCSharp Oct 11, 2026
115aa5f
Fix RemoveDst
BCSharp Oct 11, 2026
adf54cb
Cache timezone adjustment rules
BCSharp Oct 11, 2026
0a2bbee
Remove ints allocation
slozier Oct 11, 2026
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122 changes: 70 additions & 52 deletions src/core/IronPython.Modules/time.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -96,7 +96,7 @@ public static string asctime(CodeContext/*!*/ context)
=> AscTimeFormat(DateTime.Now);

public static string asctime(CodeContext/*!*/ context, [NotNone] PythonTuple time)
=> AscTimeFormat(GetDateTimeFromTupleNoDst(context, time));
=> AscTimeFormat(GetDateTimeFromTuple(time, out _));

public static double clock() {
InitStopWatch();
Expand Down Expand Up @@ -163,15 +163,32 @@ public static PythonTuple gmtime(object? seconds) {
}

public static double mktime(CodeContext/*!*/ context, [NotNone] PythonTuple localTime) {
return TicksToTimestamp(GetDateTimeFromTuple(context, localTime).AddSeconds(timezone).Ticks);
DateTime raw = GetDateTimeFromTuple(localTime, out int isDst);
TimeZoneInfo tz = TimeZoneInfo.Local;
if (tz.IsAmbiguousTime(raw)) {
// raw occurs twice (once under daylight, once under standard). Resolve directly from the
// ambiguous offset pair rather than going through RemoveDst+GetStandardOffset: raw can sit
// exactly on an AdjustmentRule's date boundary (e.g. a jurisdiction abolishing DST), where
// date-only rule matching can't tell which side of the transition the shifted result landed on.
bool preferDaylight = isDst != 0;
var offsets = tz.GetAmbiguousTimeOffsets(raw);
return TicksToTimestamp((raw - (preferDaylight ? offsets.Max() : offsets.Min())).Ticks);
}

DateTime res = isDst switch {
0 => raw,
< 0 => RemoveDst(raw),
_ => RemoveDst(raw, true),
};
return TicksToTimestamp((res - GetStandardOffset(res)).Ticks);
}

public static string strftime(CodeContext/*!*/ context, [NotNone] string format) {
return strftime(context, format, DateTime.Now, null, TimeZoneInfo.Local, errorOnF: true);
}

public static string strftime(CodeContext/*!*/ context, [NotNone] string format, [NotNone] PythonTuple dateTime) {
return strftime(context, format, GetDateTimeFromTupleNoDst(context, dateTime), null, TimeZoneInfo.Local, errorOnF: true);
return strftime(context, format, GetDateTimeFromTuple(dateTime, out _), null, TimeZoneInfo.Local, errorOnF: true);
}

public static object? strptime(CodeContext/*!*/ context, [NotNone] string @string) {
Expand Down Expand Up @@ -251,14 +268,45 @@ internal static DateTime TimestampToLocalDateTime(double timeStamp)
private static DateTime UtcToLocalDateTime(DateTime utc)
=> new DateTime(utc.Ticks + TimeZoneInfo.Local.GetUtcOffset(utc).Ticks);

// The standard (non-DST) UTC offset in effect on the given date. Unlike TimeZoneInfo.Local.BaseUtcOffset
// (a single zone-wide value), this is correct even for a jurisdiction that permanently changes its
// standard offset (e.g. abolishing DST).
private static TimeSpan GetStandardOffset(DateTime date) {
TimeZoneInfo tz = TimeZoneInfo.Local;
TimeSpan offset = tz.GetUtcOffset(date);
if (tz.IsDaylightSavingTime(date)) {
var rule = GetAdjustmentRules(tz).FirstOrDefault(r => r.DaylightDelta != TimeSpan.Zero
&& r.DateStart.Date <= date.Date && date.Date <= r.DateEnd.Date);
if (rule != null) offset -= rule.DaylightDelta;
}
return offset;
}

private sealed class AdjustmentRulesCache(TimeZoneInfo tz, TimeZoneInfo.AdjustmentRule[] rules) {
public TimeZoneInfo Tz { get; } = tz;
public TimeZoneInfo.AdjustmentRule[] Rules { get; } = rules;
}

private static AdjustmentRulesCache? _adjustmentRulesCache;

private static TimeZoneInfo.AdjustmentRule[] GetAdjustmentRules(TimeZoneInfo tz) {
var cache = _adjustmentRulesCache;
if (cache is null || !ReferenceEquals(cache.Tz, tz)) {
cache = new AdjustmentRulesCache(tz, tz.GetAdjustmentRules());
_adjustmentRulesCache = cache;
}
return cache.Rules;
}

private static DateTime RemoveDst(DateTime dt, bool always = false) {
// Use the date-aware standard offset (GetStandardOffset), not TimeZoneInfo.Local.BaseUtcOffset
if (always && TimeZoneInfo.Local.IsAmbiguousTime(dt)) {
// the time occurs twice (at the end of DST), pick the daylight saving time offset
return dt - (TimeZoneInfo.Local.GetAmbiguousTimeOffsets(dt).Max() - TimeZoneInfo.Local.BaseUtcOffset);
return dt - (TimeZoneInfo.Local.GetAmbiguousTimeOffsets(dt).Max() - GetStandardOffset(dt));
}

if (always || TimeZoneInfo.Local.IsDaylightSavingTime(dt)) {
dt -= TimeZoneInfo.Local.GetUtcOffset(dt) - TimeZoneInfo.Local.BaseUtcOffset;
dt -= TimeZoneInfo.Local.GetUtcOffset(dt) - GetStandardOffset(dt);
}

return dt;
Expand Down Expand Up @@ -423,60 +471,30 @@ internal static struct_time GetDateTimeTuple(DateTime dt, bool dstMode) {
return new struct_time(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second, Weekday(dt), dt.DayOfYear, dstMode ? 1 : 0);
}

private static DateTime GetDateTimeFromTuple(CodeContext/*!*/ context, PythonTuple t) {
DateTime res = GetDateTimeFromTupleNoDst(context, t, out int[]? ints);

if (ints != null) {
switch (ints[IsDaylightSavingsIndex]) {
// automatic detection
case -1: res = RemoveDst(res); break;
// is daylight savings time, force adjustment
case 1: res = RemoveDst(res, true); break;
}
}
return res;
}
private static int ToInt32(object? o)
=> Converter.TryConvertToIndex(o, out int i) ? i : throw PythonOps.TypeErrorForUnIndexableObject(o);

private static DateTime GetDateTimeFromTupleNoDst(CodeContext context, PythonTuple t) {
return GetDateTimeFromTupleNoDst(context, t, out _);
}
private static DateTime GetDateTimeFromTuple(PythonTuple t, out int isDst) {
if (t.__len__() != MaxIndex) throw PythonOps.TypeError("expected tuple of length {0}", MaxIndex);

private static DateTime GetDateTimeFromTupleNoDst(CodeContext context, PythonTuple t, out int[]? ints) {
if (t == null) {
ints = null;
return DateTime.Now;
}
var year = ToInt32(t[YearIndex]);
var month = ToInt32(t[MonthIndex]);
var day = ToInt32(t[DayIndex]);
var hour = ToInt32(t[HourIndex]);
var minute = ToInt32(t[MinuteIndex]);
var second = ToInt32(t[SecondIndex]);
var weekday = ToInt32(t[WeekdayIndex]);
ToInt32(t[DayOfYearIndex]); // validate
isDst = ToInt32(t[IsDaylightSavingsIndex]);

ints = ValidateDateTimeTuple(context, t);
if (year < DateTime.MinValue.Year) throw PythonOps.ValueError("year is too low");
if (year > DateTime.MaxValue.Year) throw PythonOps.ValueError("year is too high");
if (weekday < 0 || weekday >= 7) throw PythonOps.ValueError("day of week is outside of 0-6 range");

var month = ints[MonthIndex];
if (month == 0) month = 1;
var day = ints[DayIndex];
if (day == 0) day = 1;

return new DateTime(ints[YearIndex], month, day, ints[HourIndex], ints[MinuteIndex], ints[SecondIndex]);
}

private static int[] ValidateDateTimeTuple(CodeContext/*!*/ context, PythonTuple t) {
if (t.__len__() != MaxIndex) throw PythonOps.TypeError("expected tuple of length {0}", MaxIndex);

int[] ints = new int[MaxIndex];
for (int i = 0; i < MaxIndex; i++) {
ints[i] = context.LanguageContext.ConvertToInt32(t[i]);
}

int year = ints[YearIndex];
if (year >= 0 && year <= 99) {
if (year > 68) {
year += 1900;
} else {
year += 2000;
}
}
if (year < DateTime.MinValue.Year) throw PythonOps.ValueError("year is too low");
if (year > DateTime.MaxValue.Year) throw PythonOps.ValueError("year is too high");
if (ints[WeekdayIndex] < 0 || ints[WeekdayIndex] >= 7) throw PythonOps.ValueError("day of week is outside of 0-6 range");
return ints;
return new DateTime(year, month, day, hour, minute, second);
}

[MemberNotNull(nameof(sw))]
Expand Down
55 changes: 53 additions & 2 deletions tests/suite/modules/system_related/test_time.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,9 +3,10 @@
# See the LICENSE file in the project root for more information.

import datetime
import os
import time

from iptest import IronPythonTestCase, is_cli, run_test
from iptest import IronPythonTestCase, is_cli, is_windows, run_test

class TimeTest(IronPythonTestCase):
def test_strftime(self):
Expand Down Expand Up @@ -137,19 +138,69 @@ def test_localtime(self):
def test_localtime_dst(self):
# https://github.com/IronLanguages/ironpython3/issues/570
# check every hour of a year so we go through the DST transitions (if the local time zone has any)
#
# Note: time.timezone/time.altzone are computed once at process start and only hold a single
# standard/daylight offset pair, so they can't describe a zone whose standard offset itself
# changes mid-year (e.g. a jurisdiction abolishing DST); don't use them as an oracle here.
t0 = 1767225600 # 2026-01-01 00:00 UTC
for h in range(365 * 24):
t = t0 + h * 3600
lt = time.localtime(t)
self.assertEqual(time.gmtime(t - (time.altzone if lt.tm_isdst else time.timezone))[:6], lt[:6], lt)
self.assertEqual(time.mktime(lt), t, lt)
self.assertEqual(datetime.datetime.fromtimestamp(t).timetuple()[:6], lt[:6])
self.assertEqual(datetime.date.fromtimestamp(t).timetuple()[:3], lt[:3])

def test_mktime_forced_dst_no_daylight(self):
# https://github.com/IronLanguages/ironpython3/issues/570
# Forcing isdst=1 on a date that has no actual daylight saving (e.g. after a
# jurisdiction permanently abolishes DST, as America/Vancouver does on
# 2026-11-01) must give the same result as isdst=0/-1
#
# Scope the local time zone to America/Vancouver for this test alone, so it
# reproduces regardless of the machine's configured zone
old_tz = os.environ.get('TZ')
os.environ['TZ'] = 'America/Vancouver'
try:
if is_cli:
import System
System.TimeZoneInfo.ClearCachedData()
active = System.TimeZoneInfo.Local.Id == 'America/Vancouver'
else:
active = hasattr(time, 'tzset')
if active:
time.tzset()

if not active:
# TZ isn't honored at runtime on this platform (e.g. native Windows,
# where both .NET's TimeZoneInfo.Local and the CRT read the OS/registry
# setting instead): nothing to scope, so there's nothing to test here.
self.skipTest('cannot override the local time zone on this platform')

t = (2026, 12, 15, 12, 0, 0, 0, 0, 0)
self.assertEqual(time.mktime(t), time.mktime(t[:8] + (1,)))
self.assertEqual(time.mktime(t), time.mktime(t[:8] + (-1,)))
finally:
if old_tz is None:
os.environ.pop('TZ', None)
else:
os.environ['TZ'] = old_tz
if is_cli:
System.TimeZoneInfo.ClearCachedData()
elif hasattr(time, 'tzset'):
time.tzset()

def test_asctime(self):
self.assertEqual(time.asctime((2009, 9, 4, 14, 57, 11, 4, 247, 0)), 'Fri Sep 4 14:57:11 2009')
self.assertEqual(time.asctime((2009, 9, 4, 14, 57, 11, 4, 247, 1)), 'Fri Sep 4 14:57:11 2009')
self.assertEqual(time.asctime((2009, 9, 4, 14, 57, 11, 4, 247, -1)), 'Fri Sep 4 14:57:11 2009')

def test_year_lt_100(self):
t = (50, 7, 1, 12, 0, 0, 2, 182, -1)
if is_cli or not is_windows:
# years below 100 are taken literally and not mapped to 19xx/20xx
self.assertEqual(time.localtime(time.mktime(t))[:6], t[:6])
else:
self.assertRaises(OverflowError, time.mktime, t)


run_test(__name__)
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