diff --git a/frontend/src/app/core/datetime/timezone.service.spec.ts b/frontend/src/app/core/datetime/timezone.service.spec.ts
index 1fe0ae270372..45ed4a8ca168 100644
--- a/frontend/src/app/core/datetime/timezone.service.spec.ts
+++ b/frontend/src/app/core/datetime/timezone.service.spec.ts
@@ -32,6 +32,7 @@ import { PathHelperService } from 'core-app/core/path-helper/path-helper.service
 import { I18nService } from 'core-app/core/i18n/i18n.service';
 import { ConfigurationService } from 'core-app/core/config/configuration.service';
 import { TimezoneService } from 'core-app/core/datetime/timezone.service';
+import moment from 'moment-timezone';
 
 describe('TimezoneService', () => {
   const TIME = '2013-02-08T09:30:26';
@@ -44,20 +45,28 @@ describe('TimezoneService', () => {
       timezone: () => timezone,
     };
 
+    if (!timezone) {
+      vi.spyOn(moment.tz, 'guess').mockReturnValue('Etc/UTC');
+    }
+
     TestBed.configureTestingModule({
-    imports: [],
-    providers: [
+      imports: [],
+      providers: [
         { provide: I18nService, useValue: {} },
         { provide: ConfigurationService, useValue: ConfigurationServiceStub },
         PathHelperService,
         TimezoneService,
         provideHttpClient(withInterceptorsFromDi()),
-    ]
-});
+      ],
+    });
 
     timezoneService = TestBed.inject(TimezoneService);
   };
 
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
   describe('without time zone set', () => {
     beforeEach(() => {
       compile();