Refactor: centralize timezone utilities in src/utils/timezone.ts (#491)

Deduplicate resolveTimezone, resolveTimezoneId, convertEventDateTimeToISO,
and getTimezoneOffset into a single canonical module, removing copies from
EventApi, eventUtils, TimezoneSelector, EventUpdateModal, and calendarUtils.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/components/Calendar/TimezoneSelector.tsx

@@ -1,7 +1,10 @@
-import { browserDefaultTimeZone } from "@/utils/timezone";
+import {
+  browserDefaultTimeZone,
+  getTimezoneOffset,
+  resolveTimezone,
+} from "@/utils/timezone";
 import { TIMEZONES } from "@/utils/timezone-data";
 import { Button, Popover } from "@linagora/twake-mui";
-import moment from "moment";
 import { MouseEvent, useMemo, useState } from "react";
 import { useI18n } from "twake-i18n";
 import { TimezoneAutocomplete } from "../Timezone/TimezoneAutocomplete";
@@ -99,28 +102,3 @@ export function useTimeZoneList() {
     return { zones, browserTz, getTimezoneOffset };
   }, []);
 }
-
-export function resolveTimezone(tzName: string): string {
-  if (TIMEZONES.zones[tzName]) {
-    return tzName;
-  }
-  if (TIMEZONES.aliases[tzName]) {
-    return TIMEZONES.aliases[tzName].aliasTo;
-  }
-  return tzName;
-}
-
-export function getTimezoneOffset(
-  tzName: string,
-  date: Date = new Date()
-): string {
-  const fmt = new Intl.DateTimeFormat(undefined, {
-    timeZone: tzName,
-    timeZoneName: "shortOffset",
-  });
-
-  const currentDate = moment(date).isValid() ? date : new Date();
-  const parts = fmt.formatToParts(currentDate);
-  const offsetPart = parts.find((p) => p.type === "timeZoneName");
-  return offsetPart?.value.replace("GMT", "UTC") ?? "";
-}

src/components/Calendar/utils/calendarUtils.ts

@@ -1,42 +1,15 @@
-import { detectDateTimeFormat } from "@/components/Event/utils/dateTimeHelpers";
 import { refreshSingularCalendar } from "@/components/Event/utils/eventUtils";
 import { Calendar } from "@/features/Calendars/CalendarTypes";
 import { getCalendarDetailAsync } from "@/features/Calendars/services";
 import { CalendarEvent } from "@/features/Events/EventsTypes";
 import { formatDateToYYYYMMDDTHHMMSS } from "@/utils/dateUtils";
 import { extractEventBaseUuid } from "@/utils/extractEventBaseUuid";
+import { convertEventDateTimeToISO } from "@/utils/timezone";
 import { SlotLabelContentArg } from "@fullcalendar/core";
 import { ThunkDispatch } from "@reduxjs/toolkit";
 import moment from "moment-timezone";
 import { useI18n } from "twake-i18n";
 
-function convertEventDateTimeToISO(
-  datetime: string,
-  eventTimezone: string,
-  isAllDay: boolean
-): string {
-  if (!datetime || isAllDay) return datetime;
-
-  if (datetime.includes("Z") || datetime.match(/[+-]\d{2}:\d{2}$/)) {
-    return datetime;
-  }
-
-  const dateOnlyRegex = /^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;
-  if (dateOnlyRegex.test(datetime)) {
-    return datetime;
-  }
-
-  const format = detectDateTimeFormat(datetime);
-  const momentDate = moment.tz(datetime, format, eventTimezone);
-  if (!momentDate.isValid()) {
-    console.warn(
-      `[convertEventDateTimeToISO] Invalid datetime: "${datetime}" with format "${format}" in timezone "${eventTimezone}"`
-    );
-    return datetime;
-  }
-  return momentDate.toISOString();
-}
-
 export const updateSlotLabelVisibility = (
   currentTime: Date,
   slotLabel: SlotLabelContentArg,
@@ -114,22 +87,18 @@ export const eventToFullCalendarFormat = (
       };
 
       if (!isAllDay && e.start && eventTimezone) {
-        const startISO = convertEventDateTimeToISO(
-          e.start,
-          eventTimezone,
-          isAllDay
-        );
+        const startISO = convertEventDateTimeToISO(e.start, eventTimezone, {
+          isAllDay,
+        });
         if (startISO) {
           convertedEvent.start = startISO;
         }
       }
 
       if (!isAllDay && e.end && eventTimezone) {
-        const endISO = convertEventDateTimeToISO(
-          e.end,
-          eventTimezone,
-          isAllDay
-        );
+        const endISO = convertEventDateTimeToISO(e.end, eventTimezone, {
+          isAllDay,
+        });
         if (endISO) {
           convertedEvent.end = endISO;
         }