Allow user to set timezone (#12775)

* Allow user to set timezone

* Update test snapshots

---------

Co-authored-by: Florian Duros <florianduros@element.io>

src/TimezoneHandler.ts

@@ -0,0 +1,55 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { SettingLevel } from "./settings/SettingLevel";
+import SettingsStore from "./settings/SettingsStore";
+
+export const USER_TIMEZONE_KEY = "userTimezone";
+
+/**
+ * Returning `undefined` ensure that if unset the browser default will be used in `DateTimeFormat`.
+ * @returns The user specified timezone or `undefined`
+ */
+export function getUserTimezone(): string | undefined {
+    const tz = SettingsStore.getValueAt(SettingLevel.DEVICE, USER_TIMEZONE_KEY);
+    return tz || undefined;
+}
+
+/**
+ * Set in the settings the given timezone
+ * @timezone
+ */
+export function setUserTimezone(timezone: string): Promise<void> {
+    return SettingsStore.setValue(USER_TIMEZONE_KEY, null, SettingLevel.DEVICE, timezone);
+}
+
+/**
+ * Return all the available timezones
+ */
+export function getAllTimezones(): string[] {
+    return Intl.supportedValuesOf("timeZone");
+}
+
+/**
+ * Return the current timezone in a short human readable way
+ */
+export function shortBrowserTimezone(): string {
+    return (
+        new Intl.DateTimeFormat(undefined, { timeZoneName: "short" })
+            .formatToParts(new Date())
+            .find((x) => x.type === "timeZoneName")?.value ?? "GMT"
+    );
+}