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fix: VTimeZone.FromSystemTimeZone for southern-hemisphere time zones #1013

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Merged
axunonb merged 4 commits into from
Oct 19, 2026
Merged

fix: VTimeZone.FromSystemTimeZone for southern-hemisphere time zones #1013

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b2c28e8
fix: `VTimeZone.FromSystemTimeZone` for southern-hemisphere time zones
axunonb Oct 17, 2026
cbae922
Add more test cases, including negative offset
axunonb Oct 17, 2026
81ac02c
Simplify VTimeZoneSouthernHemisphereDaylightShouldSerializeProperly
axunonb Oct 18, 2026
cf5c042
Rename test method
axunonb Oct 18, 2026
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36 changes: 36 additions & 0 deletions Ical.Net.Tests/VTimeZoneTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -135,6 +135,42 @@ public void VTimeZoneEuropeMoscowShouldSerializeProperly()
}
}

[TestCase("Australia/Sydney", "+1000", "+1100", "T020000")]
[TestCase("Australia/Lord_Howe", "+1030", "+1100", "T020000")]
[TestCase("Australia/Adelaide", "+0930", "+1030", "T020000")]
[TestCase("Australia/Broken_Hill", "+0930", "+1030", "T020000")]
[TestCase("Pacific/Auckland", "+1200", "+1300", "T020000")]
[TestCase("Pacific/Easter", "-0600", "-0500", "T220000")]
[TestCase("Pacific/Chatham", "+1245", "+1345", "T024500")]
[TestCase("America/Santiago", "-0400", "-0300", "T000000")]
[TestCase("Antarctica/Macquarie", "+1000", "+1100", "T020000")]
[Category("VTimeZone")]
public void VTimeZoneSouthernHemisphereDetermineDaylight(string tzId, string expectedOffsetFrom,
string expectedOffsetTo, string expectedDtStartTime)
{
TimeZoneInfo tzInfo;
try
{
tzInfo = TimeZoneInfo.FindSystemTimeZoneById(tzId);
}
catch (TimeZoneNotFoundException)
{
Assert.Ignore($"Time zone '{tzId}' was not found on the local machine.");
return;
}

var tz = VTimeZone.FromSystemTimeZone(tzInfo, new DateTime(2024, 1, 1), false);

@maknapp maknapp Oct 18, 2026

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Could use VTimeZone.FromDateTimeZone(tzId, ...) directly. Both methods just use the TZID to get the NodaTime time zone anyway.

var daylight = tz.TimeZoneInfos.FirstOrDefault(x => x.Name == "DAYLIGHT")!;

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using (Assert.EnterMultipleScope())
{
Assert.That(daylight, Is.Not.Null);
Assert.That(daylight.OffsetFrom?.ToString(), Is.EqualTo(expectedOffsetFrom));
Assert.That(daylight.OffsetTo?.ToString(), Is.EqualTo(expectedOffsetTo));
Assert.That(daylight.DtStart!.ToBasicIso(), Does.EndWith(expectedDtStartTime));
}
}

[Test, Category("VTimeZone")]
public void VTimeZoneAmericaChicagoShouldSerializeProperly()
{
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19 changes: 13 additions & 6 deletions Ical.Net/CalendarComponents/VTimeZone.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -96,7 +96,7 @@ internal static VTimeZone FromDateTimeZone(
wallOffset: tz.MinOffset,
savings: Offset.Zero);
intervals.Add(interval);
var zoneInfo = CreateTimeZoneInfo(intervals, [], true, true);
var zoneInfo = CreateTimeZoneInfo(tz, intervals, [], true, true);
vTimeZone.AddChild(zoneInfo);
}
else
Expand All @@ -114,7 +114,7 @@ internal static VTimeZone FromDateTimeZone(
}

matchingStandardIntervals = GetMatchingIntervals(standardIntervals, latestStandardInterval, true);
var latestStandardTimeZoneInfo = CreateTimeZoneInfo(matchingStandardIntervals, intervals);
var latestStandardTimeZoneInfo = CreateTimeZoneInfo(tz, matchingStandardIntervals, intervals);
vTimeZone.AddChild(latestStandardTimeZoneInfo);

// check to see if there is no active, future daylight savings (ie, America/Phoenix)
Expand All @@ -128,7 +128,7 @@ internal static VTimeZone FromDateTimeZone(
var latestDaylightInterval = daylightIntervals.OrderByDescending(x => x.Start).First();
matchingDaylightIntervals =
GetMatchingIntervals(daylightIntervals, latestDaylightInterval, true);
var latestDaylightTimeZoneInfo = CreateTimeZoneInfo(matchingDaylightIntervals, intervals);
var latestDaylightTimeZoneInfo = CreateTimeZoneInfo(tz, matchingDaylightIntervals, intervals);
vTimeZone.AddChild(latestDaylightTimeZoneInfo);
}
}
Expand All @@ -152,15 +152,15 @@ internal static VTimeZone FromDateTimeZone(
}

var matchedIntervals = GetMatchingIntervals(historicIntervals, interval);
var timeZoneInfo = CreateTimeZoneInfo(matchedIntervals, intervals, false);
var timeZoneInfo = CreateTimeZoneInfo(tz, matchedIntervals, intervals, false);
vTimeZone.AddChild(timeZoneInfo);
historicIntervals = historicIntervals.Where(x => !matchedIntervals.Contains(x)).ToList();
}

return vTimeZone;
}

private static VTimeZoneInfo CreateTimeZoneInfo(List<ZoneInterval> matchedIntervals, List<ZoneInterval> intervals, bool isRRule = true,
private static VTimeZoneInfo CreateTimeZoneInfo(DateTimeZone tz, List<ZoneInterval> matchedIntervals, List<ZoneInterval> intervals, bool isRRule = true,
bool isOnlyInterval = false)
{
if (matchedIntervals == null || matchedIntervals.Count == 0)
Expand All @@ -176,7 +176,7 @@ private static VTimeZoneInfo CreateTimeZoneInfo(List<ZoneInterval> matchedInterv

var previousInterval = intervals.SingleOrDefault(x => (x.HasEnd ? x.End : Instant.MaxValue) == oldestInterval.Start);

var delta = new TimeSpan(1, 0, 0);
TimeSpan delta;

if (previousInterval != null)
{
Expand All @@ -186,6 +186,13 @@ private static VTimeZoneInfo CreateTimeZoneInfo(List<ZoneInterval> matchedInterv
{
delta = TimeSpan.Zero;
}
else
{
// The true predecessor was not collected in the interval range (e.g. southern-hemisphere
// zones where Jan 1st falls inside DST). Look it up directly from the time zone itself.
var actualPrevious = tz.GetZoneInterval(oldestInterval.Start - NodaTime.Duration.Epsilon);
delta = new TimeSpan(0, 0, actualPrevious.WallOffset.Seconds - oldestInterval.WallOffset.Seconds);
}

var utcOffset = oldestInterval.StandardOffset.ToTimeSpan();

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