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/utils/timezone.ts

@@ -2,6 +2,8 @@
 
 // Ensure ICAL is imported before using this module.
 import ICAL from "ical.js";
+import moment from "moment-timezone";
+import { detectDateTimeFormat } from "@/components/Event/utils/dateTimeHelpers";
 
 // TIMEZONES data must be imported or defined separately.
 import { TIMEZONES } from "./timezone-data";
@@ -39,3 +41,61 @@ function findTimezone(tzid: string): any {
 
   throw new Error(`Unknown timezone alias: ${tzid}`);
 }
+
+export function resolveTimezone(tzName: string): string {
+  if (TIMEZONES.zones[tzName]) {
+    return tzName;
+  }
+  if (TIMEZONES.aliases[tzName]) {
+    return TIMEZONES.aliases[tzName].aliasTo;
+  }
+  return tzName;
+}
+
+export function resolveTimezoneId(tzid?: string): string | undefined {
+  if (!tzid) return undefined;
+  return resolveTimezone(tzid);
+}
+
+export function convertEventDateTimeToISO(
+  datetime: string,
+  timezone: string,
+  options?: { isAllDay?: boolean }
+): string | undefined {
+  if (!datetime || !timezone) return undefined;
+  if (options?.isAllDay) return undefined;
+
+  if (datetime.includes("Z") || datetime.match(/[+-]\d{2}:\d{2}$/)) {
+    return datetime;
+  }
+
+  const dateOnlyRegex = /^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;
+  if (dateOnlyRegex.test(datetime)) {
+    return undefined;
+  }
+
+  const format = detectDateTimeFormat(datetime);
+  const momentDate = moment.tz(datetime, format, timezone);
+  if (!momentDate.isValid()) {
+    console.warn(
+      `[convertEventDateTimeToISO] Invalid datetime: "${datetime}" with format "${format}" in timezone "${timezone}"`
+    );
+    return undefined;
+  }
+  return momentDate.toISOString();
+}
+
+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") ?? "";
+}