diff --git a/frontend/src/components/tracker/SleepButton.tsx b/frontend/src/components/tracker/SleepButton.tsx
index 8d80fac..0e781b6 100644
--- a/frontend/src/components/tracker/SleepButton.tsx
+++ b/frontend/src/components/tracker/SleepButton.tsx
@@ -10,6 +10,10 @@ const SleepButton: React.FC<SleepButtonProps> = ({ time }) => {
   const isSleeping = !!activeSession;
 
   const handleClick = async () => {
+    if (!time) {
+      alert('Please select a valid time.');
+      return;
+    }
     try {
       if (isSleeping && activeSession?.id) {
         await endSession({ sessionId: activeSession.id, endTime: time });
diff --git a/frontend/src/utils/time.ts b/frontend/src/utils/time.ts
index a06b79f..2a2cff8 100644
--- a/frontend/src/utils/time.ts
+++ b/frontend/src/utils/time.ts
@@ -90,6 +90,9 @@ export function formatDateTime(date: Date | string): string {
  */
 export function toUTCFromLocalInput(inputValue: string): string {
   const d = new Date(inputValue);
+  if (isNaN(d.getTime())) {
+    return "";
+  }
   return d.toISOString();
 }
 
diff --git a/frontend/tests/unit/SleepButton.test.tsx b/frontend/tests/unit/SleepButton.test.tsx
index f5a4a1d..7efbd0d 100644
--- a/frontend/tests/unit/SleepButton.test.tsx
+++ b/frontend/tests/unit/SleepButton.test.tsx
@@ -70,4 +70,20 @@ describe('SleepButton', () => {
 
     await waitFor(() => expect(endSessionMock).toHaveBeenCalled());
   });
+
+  it('shows alert when time is invalid', async () => {
+    const alertSpy = vi.spyOn(window, 'alert').mockImplementation(() => {});
+    (useSleepSession as any).mockReturnValue({
+      activeSession: null,
+      isCreating: false,
+      isEnding: false,
+    });
+
+    renderWithProviders(<SleepButton time="" />);
+    const button = screen.getByRole('button', { name: /Start Sleep/i });
+    fireEvent.click(button);
+
+    expect(alertSpy).toHaveBeenCalledWith('Please select a valid time.');
+    alertSpy.mockRestore();
+  });
 });
diff --git a/frontend/tests/unit/TimeAdjuster.test.tsx b/frontend/tests/unit/TimeAdjuster.test.tsx
index 4a1d5a4..c7448b6 100644
--- a/frontend/tests/unit/TimeAdjuster.test.tsx
+++ b/frontend/tests/unit/TimeAdjuster.test.tsx
@@ -29,4 +29,18 @@ describe('TimeAdjuster', () => {
     fireEvent.click(resetButton);
     expect(handleChange).toHaveBeenCalled();
   });
+
+  it('handles invalid date input gracefully (returns empty string)', async () => {
+    // Since we changed the implementation, it should no longer throw.
+    // We can't easily test the useEffect error anymore because it doesn't throw,
+    // but we can check if onTimeChange is called with "" for invalid input.
+    const handleChange = vi.fn();
+    const { container } = render(<TimeAdjuster onTimeChange={handleChange} />);
+    const input = container.querySelector('input');
+
+    fireEvent.change(input!, { target: { value: '2026-06-0' } });
+    // We might need to wait for the effect
+    await new Promise((r) => setTimeout(r, 0));
+    expect(handleChange).toHaveBeenCalledWith("");
+  });
 });
diff --git a/frontend/tests/unit/time.test.ts b/frontend/tests/unit/time.test.ts
index a2ef6be..5362c55 100644
--- a/frontend/tests/unit/time.test.ts
+++ b/frontend/tests/unit/time.test.ts
@@ -12,6 +12,7 @@ import {
   adjustForTimezone,
   getLocalOffsetFromReference,
   formatDateTime,
+  toUTCFromLocalInput,
 } from '../../src/utils/time';
 
 describe('formatMinutesAfter', () => {
@@ -152,3 +153,18 @@ describe('getLocalOffsetFromReference', () => {
     expect(offset).toBe(localMins - refMins);
   });
 });
+
+describe('toUTCFromLocalInput', () => {
+  it('converts valid datetime-local string to ISO UTC string', () => {
+    const input = '2024-01-15T10:00';
+    // This is tricky because it depends on local timezone. 
+    // But we just want to see if it works for a valid input.
+    const result = toUTCFromLocalInput(input);
+    expect(result).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}Z$/);
+  });
+
+  it('returns empty string on invalid datetime-local string', () => {
+    expect(toUTCFromLocalInput('2026-06-0')).toBe('');
+  });
+});
+