test: add failing TDD test for system time checkbox behavior

src/__tests__/TimeConverter.test.tsx

@@ -66,4 +66,45 @@ describe("TimeConverter", () => {
       new Date(2025, 0, 1, 12, 0, 0),
     );
   });
+
+  // ──────────────────────────────────────────────────────────
+  // NEW TDD TEST – should FAIL with current component code
+  // ──────────────────────────────────────────────────────────
+  it("switches to system time when 'Use system time' checkbox is checked (TDD)", () => {
+    // Fix system time to 15:00:00
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2025, 0, 1, 15, 0, 0));
+
+    // A mutable currentTime that the mock will return
+    let currentDate = new Date(2025, 0, 1, 12, 0, 0);
+    const setCurrentDate = vi.fn((d: Date) => {
+      currentDate = d;
+    });
+
+    mockedUseCurrentTime.mockImplementation(() => ({
+      currentTime: currentDate,
+      setCurrentTime: setCurrentDate,
+    }));
+
+    render(<TimeConverter />);
+
+    // Initially the digital clock shows the manual time 12:00:00
+    expect(screen.getByTestId("digital-clock-time")).toHaveTextContent(
+      new Date(2025, 0, 1, 12, 0, 0).toISOString(),
+    );
+
+    // Click the "Use system time" checkbox
+    fireEvent.click(screen.getByTestId("use-system-time"));
+
+    // The component SHOULD have called setCurrentDate with the system time
+    // (currently it does NOT, so this assertion fails)
+    expect(setCurrentDate).toHaveBeenCalledWith(new Date(2025, 0, 1, 15, 0, 0));
+
+    // And the digital clock should now show the system time 15:00:00
+    expect(screen.getByTestId("digital-clock-time")).toHaveTextContent(
+      new Date(2025, 0, 1, 15, 0, 0).toISOString(),
+    );
+
+    vi.useRealTimers();
+  });
 });