Only set timezone when user setting is a valid timezone

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

@@ -21,6 +21,7 @@ export const momentMock = {
   locale: jest.fn(() => 'default-locale'),
   tz: {
     setDefault: jest.fn(),
+    names: jest.fn(() => ['tz1', 'tz2', 'tz3']),
   },
   weekdays: jest.fn(() => ['dow1', 'dow2', 'dow3']),
   updateLocale: jest.fn(),

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

@@ -47,6 +47,30 @@ 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('tz4');
+    const dow$ = new BehaviorSubject('dow1');
+
+    const uiSettings = uiSettingsServiceMock.createSetupContract();
+    uiSettings.get$.mockReturnValueOnce(tz$).mockReturnValueOnce(dow$);
+
+    service.start({ uiSettings });
+    await flushPromises();
+    expect(momentMock.tz.setDefault).not.toHaveBeenCalledWith('tz4');
+  });
+
+  it('sets timezone when a zone is defined', async () => {
+    const tz$ = new BehaviorSubject('tz3');
+    const dow$ = new BehaviorSubject('dow1');
+
+    const uiSettings = uiSettingsServiceMock.createSetupContract();
+    uiSettings.get$.mockReturnValueOnce(tz$).mockReturnValueOnce(dow$);
+
+    service.start({ uiSettings });
+    await flushPromises();
+    expect(momentMock.tz.setDefault).toHaveBeenCalledWith('tz3');
+  });
+
   test('updates moment config', async () => {
     const tz$ = new BehaviorSubject('tz1');
     const dow$ = new BehaviorSubject('dow1');

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

@@ -35,7 +35,15 @@ 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 timezones = moment.tz.names();
+      const hasTimezone = timezones.includes(tz);
+      if (hasTimezone) {
+        moment.tz.setDefault(tz);
+      } else {
+        moment.tz.setDefault();
+      }
+    };
     const setStartDayOfWeek = (day: string) => {
       const dow = moment.weekdays().indexOf(day);
       moment.updateLocale(moment.locale(), { week: { dow } } as any);