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fix(ical): honor elapsed duration across daylight saving changes #566

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Merged
KnifeLemon merged 1 commit into from
Oct 25, 2026
Merged

fix(ical): honor elapsed duration across daylight saving changes #566

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43 changes: 38 additions & 5 deletions src/Filee.Engines/Data/ICalendar.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -226,6 +226,21 @@ private static bool ParseIcsDate(Property property, out DateTime value, out bool
catch (InvalidTimeZoneException) { return null; }
}

// Resolve the offset at an instant after adding the clock part of a DURATION. The local
// wall time may have crossed a daylight-saving transition since DTSTART.
private static TimeSpan? OffsetAtInstant(string id, DateTimeOffset instant, Dictionary<string, ZoneDefinition> zones)
{
if (zones.TryGetValue(id, out var definition) && !definition.HasRule && definition.Transitions.Count > 0)
{
var ordered = definition.Transitions.OrderBy(t => new DateTimeOffset(t.Local, t.From)).ToList();
var found = ordered.LastOrDefault(t => new DateTimeOffset(t.Local, t.From) <= instant);
return found == default ? ordered[0].From : found.To;
}
try { return TimeZoneInfo.FindSystemTimeZoneById(id).GetUtcOffset(instant); }
catch (TimeZoneNotFoundException) { return null; }
catch (InvalidTimeZoneException) { return null; }
}

private static string DateCell(Property? property, Dictionary<string, ZoneDefinition> zones)
{
if (property is null)
Expand All @@ -242,7 +257,9 @@ private static string DateCell(Property? property, Dictionary<string, ZoneDefini
return offset is null ? text : new DateTimeOffset(value, offset.Value).ToString("yyyy-MM-dd'T'HH:mm:sszzz", CultureInfo.InvariantCulture);
}

private static bool Duration(string text, out TimeSpan duration)
private readonly record struct CalendarDuration(TimeSpan Days, TimeSpan Clock);

private static bool Duration(string text, out CalendarDuration duration)
{
duration = default;
var sign = 1;
Expand All @@ -257,6 +274,8 @@ private static bool Duration(string text, out TimeSpan duration)
var time = false;
var digits = "";
var any = false;
var days = TimeSpan.Zero;
var clock = TimeSpan.Zero;
foreach (var ch in text)
{
if (ch == 'T' && !time)
Expand All @@ -271,7 +290,7 @@ private static bool Duration(string text, out TimeSpan duration)
}
if (digits.Length == 0 || !int.TryParse(digits, out var number))
return false;
duration += ch switch
var part = ch switch
{
'W' when !time => TimeSpan.FromDays(number * 7d),
'D' when !time => TimeSpan.FromDays(number),
Expand All @@ -280,12 +299,16 @@ private static bool Duration(string text, out TimeSpan duration)
'S' when time => TimeSpan.FromSeconds(number),
_ => TimeSpan.MinValue,
};
if (duration < TimeSpan.Zero)
if (part < TimeSpan.Zero)
return false;
if (ch is 'W' or 'D')
days += part;
else
clock += part;
digits = "";
any = true;
}
duration *= sign;
duration = new CalendarDuration(days * sign, clock * sign);
return any && digits.Length == 0;
}

Expand All @@ -300,10 +323,20 @@ public static List<List<string>> ToRows(List<Component> components)
if (end is null && start is not null && ParseIcsDate(start, out var startDate, out var date, out var utc)
&& component.First("DURATION") is { } span && Duration(span.Value, out var duration))
{
var computed = startDate + duration;
// Calendar days keep the local wall time; hours, minutes and seconds are elapsed time (RFC 5545).
var dayEnd = startDate + duration.Days;
var computed = dayEnd + duration.Clock;
var synthetic = new Property(start.Name, computed.ToString(date ? "yyyyMMdd" : "yyyyMMdd'T'HHmmss", CultureInfo.InvariantCulture)
+ (utc && !date ? "Z" : ""), "", start.Parameters);
endCell = DateCell(synthetic, component.Zones);
var zone = start.Parameter("TZID");
if (!date && !utc && duration.Clock != TimeSpan.Zero && zone.Length > 0
&& Offset(zone, dayEnd, component.Zones) is { } dayOffset)
{
var instant = new DateTimeOffset(dayEnd, dayOffset).Add(duration.Clock);
if (OffsetAtInstant(zone, instant, component.Zones) is { } endOffset)
endCell = instant.ToOffset(endOffset).ToString("yyyy-MM-dd'T'HH:mm:sszzz", CultureInfo.InvariantCulture);
}
}
rows.Add(
[
Expand Down
20 changes: 20 additions & 0 deletions tests/Filee.Engines.Tests/ICalendarTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,26 @@ public async Task Apple_all_day_exclusive_end_duration_and_unknown_properties()
Assert.Equal("2026-11-04", Cell(rows, 3, "End"));
}

[Fact]
public async Task Duration_crossing_daylight_saving_time_keeps_calendar_days_and_elapsed_hours()
{
var customZone = string.Join("\r\n", "BEGIN:VTIMEZONE", "TZID:Custom/Spring", "BEGIN:DAYLIGHT",
"DTSTART:20260308T020000", "TZOFFSETFROM:-0500", "TZOFFSETTO:-0400", "END:DAYLIGHT", "END:VTIMEZONE");
var rows = await ReadCalendar(Calendar(customZone,
Event("spring-clock", "DTSTART;TZID=America/New_York:20260308T013000", "DURATION:PT2H"),
Event("spring-day", "DTSTART;TZID=America/New_York:20260308T013000", "DURATION:P1D"),
Event("spring-24h", "DTSTART;TZID=America/New_York:20260308T013000", "DURATION:PT24H"),
Event("spring-mixed", "DTSTART;TZID=America/New_York:20260307T013000", "DURATION:P1DT2H"),
Event("fall-clock", "DTSTART;TZID=America/New_York:20261101T003000", "DURATION:PT2H"),
Event("custom", "DTSTART;TZID=Custom/Spring:20260308T013000", "DURATION:PT2H")));
Assert.Equal("2026-03-08T04:30:00-04:00", Cell(rows, 1, "End"));
Assert.Equal("2026-03-09T01:30:00-04:00", Cell(rows, 2, "End"));
Assert.Equal("2026-03-09T02:30:00-04:00", Cell(rows, 3, "End"));
Assert.Equal("2026-03-08T04:30:00-04:00", Cell(rows, 4, "End"));
Assert.Equal("2026-11-01T01:30:00-05:00", Cell(rows, 5, "End"));
Assert.Equal("2026-03-08T04:30:00-04:00", Cell(rows, 6, "End"));
}

[Fact]
public async Task Outlook_windows_zone_custom_zone_unknown_rules_and_recurrence_instance()
{
Expand Down
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