fix(web): handling historical timezones in web client (#18905)

* fix handling historical timezones in web client

* honor dst when calculating the timezone offset

* fix variable used to construct timezones list to honor dst

* remove unused variable. fix lint

web/src/lib/components/shared-components/change-date.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import ConfirmModal from '$lib/modals/ConfirmModal.svelte';
-  import { DateTime } from 'luxon';
+  import { DateTime, Duration } from 'luxon';
   import { t } from 'svelte-i18n';
   import DateInput from '../elements/date-input.svelte';
   import Combobox, { type ComboBoxOption } from './combobox.svelte';
@@ -65,6 +65,9 @@
   const userTimeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
 
   let selectedDate = $state(initialDate.toFormat("yyyy-MM-dd'T'HH:mm"));
+  // Use a fixed modern date to calculate stable timezone offsets for the list
+  // This ensures that the offsets shown in the combobox are always current,
+  // regardless of the historical date selected by the user.
   let timezones: ZoneOption[] = knownTimezones
     .map((zone) => zoneOptionForDate(zone, selectedDate))
     .filter((zone) => zone.valid)
@@ -73,12 +76,13 @@
   let selectedOption: ZoneOption | undefined = $state(getPreferredTimeZone(initialDate, userTimeZone, timezones));
 
   function zoneOptionForDate(zone: string, date: string) {
-    const dateAtZone: DateTime = DateTime.fromISO(date, { zone });
+    const { offsetMinutes, offsetFormat: zoneOffsetAtDate } = getModernOffsetForZoneAndDate(zone, date);
-    const zoneOffsetAtDate = dateAtZone.toFormat('ZZ');
+    // For validity, we still need to check if the exact date/time exists in the *original* timezone (for gaps/overlaps).
-    const valid = dateAtZone.isValid && date.toString() === dateAtZone.toFormat("yyyy-MM-dd'T'HH:mm");
+    const dateForValidity = DateTime.fromISO(date, { zone });
+    const valid = dateForValidity.isValid && date === dateForValidity.toFormat("yyyy-MM-dd'T'HH:mm");
     return {
       value: zone,
-      offsetMinutes: dateAtZone.offset,
+      offsetMinutes,
       label: zone + ' (' + zoneOffsetAtDate + ')' + (valid ? '' : ' [invalid date!]'),
       valid,
     };
@@ -118,6 +122,24 @@
     return previousSelection ?? fromInitialTimeZone ?? sameAsUserTimeZone ?? firstWithSameOffset ?? utcFallback;
   }
 
+  function getModernOffsetForZoneAndDate(
+    zone: string,
+    dateString: string,
+  ): { offsetMinutes: number; offsetFormat: string } {
+    const dt = DateTime.fromISO(dateString, { zone });
+
+    // we determine the *modern* offset for this zone based on its current rules.
+    // To do this, we "move" the date to the current year, keeping the local time components.
+    // This allows Luxon to apply current-year DST rules.
+    const modernYearDt = dt.set({ year: DateTime.now().year });
+
+    // Calculate the offset at that modern year's date.
+    const modernOffsetMinutes = modernYearDt.setZone(zone, { keepLocalTime: true }).offset;
+    const modernOffsetFormat = modernYearDt.setZone(zone, { keepLocalTime: true }).toFormat('ZZ');
+
+    return { offsetMinutes: modernOffsetMinutes, offsetFormat: modernOffsetFormat };
+  }
+
   function sortTwoZones(zoneA: ZoneOption, zoneB: ZoneOption) {
     let offsetDifference = zoneA.offsetMinutes - zoneB.offsetMinutes;
     if (offsetDifference != 0) {
@@ -127,9 +149,20 @@
   }
 
   const handleConfirm = () => {
-    const value = date.toISO();
+    if (date.isValid && selectedOption) {
-    if (value) {
+      // Get the local date/time components from the selected string using neutral timezone
-      onConfirm(value);
+      const dtComponents = DateTime.fromISO(selectedDate, { zone: 'utc' });
+
+      // Determine the modern, DST-aware offset for the selected IANA zone
+      const { offsetMinutes } = getModernOffsetForZoneAndDate(selectedOption.value, selectedDate);
+
+      // Construct the final ISO string with a fixed-offset zone.
+      const fixedOffsetZone = `UTC${offsetMinutes >= 0 ? '+' : ''}${Duration.fromObject({ minutes: offsetMinutes }).toFormat('hh:mm')}`;
+
+      // Create a DateTime object in this fixed-offset zone, preserving the local time.
+      const finalDateTime = DateTime.fromObject(dtComponents.toObject(), { zone: fixedOffsetZone });
+
+      onConfirm(finalDateTime.toISO({ includeOffset: true })!);
     }
   };