Only set timezone when user setting is a valid timezone

src/core/public/integrations/moment/moment_service.test.mocks.ts

@@ -21,6 +21,9 @@ export const momentMock = {
   locale: jest.fn(() => 'default-locale'),
   tz: {
     setDefault: jest.fn(),
+    zone: jest.fn(
+      z => [{ name: 'tz1' }, { name: 'tz2' }, { name: 'tz3' }].find(f => z === f.name) || null
+    ),
   },
   weekdays: jest.fn(() => ['dow1', 'dow2', 'dow3']),
   updateLocale: jest.fn(),

src/core/public/integrations/moment/moment_service.test.ts

@@ -47,6 +47,26 @@ describe('MomentService', () => {
     expect(momentMock.updateLocale).toHaveBeenCalledWith('default-locale', { week: { dow: 0 } });
   });
 
+  it('uses the default timezone when a zone is not defined', async () => {
+    const tz$ = new BehaviorSubject('timezone/undefined');
+    const uiSettings = uiSettingsServiceMock.createSetupContract();
+    uiSettings.get$.mockReturnValueOnce(tz$);
+
+    service.start({ uiSettings });
+    await flushPromises();
+    expect(momentMock.tz.setDefault).not.toHaveBeenCalled();
+  });
+
+  it('sets timezone when a zone is defined', async () => {
+    const tz$ = new BehaviorSubject('tz3');
+    const uiSettings = uiSettingsServiceMock.createSetupContract();
+    uiSettings.get$.mockReturnValueOnce(tz$);
+
+    service.start({ uiSettings });
+    await flushPromises();
+    expect(momentMock.tz.setDefault).toHaveBeenCalledWith('tz3');
+  });
+
   test('updates moment config', async () => {
     const tz$ = new BehaviorSubject('tz1');
     const dow$ = new BehaviorSubject('dow1');

src/core/public/integrations/moment/moment_service.ts

@@ -35,7 +35,10 @@ export class MomentService implements CoreService {
   public async setup() {}
 
   public async start({ uiSettings }: StartDeps) {
-    const setDefaultTimezone = (tz: string) => moment.tz.setDefault(tz);
+    const setDefaultTimezone = (tz: string) => {
+      const zone = moment.tz.zone(tz);
+      if (zone) moment.tz.setDefault(zone.name);
+    };
     const setStartDayOfWeek = (day: string) => {
       const dow = moment.weekdays().indexOf(day);
       moment.updateLocale(moment.locale(), { week: { dow } } as any);