test: fix timezone issues in EventDisplay tests

- Set explicit UTC timezone globally in jest.config.ts and setupTests.ts
- Mock Intl.DateTimeFormat to use UTC by default while preserving prototype
- Replace regex patterns with exact values for declarative assertions
- Remove complex date formatting computations in tests
- Use fixed dates (2025-01-15T10:00:00.000Z) for consistent test results
- Fix EventDisplay.test.tsx to use exact time values instead of regex

src/setupTests.ts

@@ -4,6 +4,33 @@ import { TextEncoder } from "util";
 
 global.TextEncoder = TextEncoder;
 
+// Set timezone to UTC for consistent test results across all environments
+process.env.TZ = "UTC";
+
+// Mock Intl.DateTimeFormat to use UTC timezone by default
+const OriginalDateTimeFormat = Intl.DateTimeFormat;
+
+// Create a constructor function that preserves prototype
+const MockedDateTimeFormat: any = function (
+  this: any,
+  locales?: string | string[],
+  options?: Intl.DateTimeFormatOptions
+) {
+  // Only set timeZone to UTC if not explicitly specified
+  const modifiedOptions = options
+    ? { timeZone: "UTC", ...options } // timeZone comes first so options can override it
+    : { timeZone: "UTC" };
+  return new OriginalDateTimeFormat(locales, modifiedOptions);
+};
+
+// Preserve the prototype so jest.spyOn works on methods
+MockedDateTimeFormat.prototype = OriginalDateTimeFormat.prototype;
+
+// Preserve static methods
+Object.setPrototypeOf(MockedDateTimeFormat, OriginalDateTimeFormat);
+
+global.Intl.DateTimeFormat = MockedDateTimeFormat;
+
 jest.mock("openid-client", () => ({
   discovery: jest.fn(),
   randomPKCECodeVerifier: jest.fn(),