diff --git a/backend/src/utils/date.ts b/backend/src/utils/date.ts
new file mode 100644
index 0000000..792bb93
--- /dev/null
+++ b/backend/src/utils/date.ts
@@ -0,0 +1,27 @@
+// Extending the built-in Date interface to give a precise return type to toISOString
+declare global {
+  interface Date {
+    toISOString(): TDateISO;
+  }
+}
+
+// Type aliases for different parts of a date/time string
+type TYear = `${number}${number}${number}${number}`;
+type TMonth = `${number}${number}`;
+type TDay = `${number}${number}`;
+type THours = `${number}${number}`;
+type TMinutes = `${number}${number}`;
+type TSeconds = `${number}${number}`;
+type TMilliseconds = `${number}${number}${number}`;
+
+// Represent a string like `2021-01-08` (date only)
+type TDateISODate = `${TYear}-${TMonth}-${TDay}`;
+
+// Represent a string like `14:42:34.678` (time part)
+type TDateISOTime = `${THours}:${TMinutes}:${TSeconds}.${TMilliseconds}`;
+
+// Represent a string like `2021-01-08T14:42:34.678Z` (full ISO 8601 format)
+type TDateISOFull = `${TDateISODate}T${TDateISOTime}Z`;
+
+// Expects either the full date-time format or date-only format
+export type TDateISO = TDateISOFull | TDateISODate;
diff --git a/backend/tests/date.test.ts b/backend/tests/date.test.ts
new file mode 100644
index 0000000..5522c6c
--- /dev/null
+++ b/backend/tests/date.test.ts
@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { TDateISO } from '../src/utils/date';  // Import the type
+
+// Helper function to check if a string matches the YYYY-MM-DD or YYYY-MM-DDTHH:mm:ss.sssZ format
+const isDateISO = (date: string): boolean => {
+  // Regex for the format YYYY-MM-DD or YYYY-MM-DDTHH:mm:ss.sssZ
+  const regex = /^(?:\d{4}-\d{2}-\d{2}|\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z)$/;
+  return regex.test(date);
+};
+
+describe('Date ISO Format', () => {
+  it('should match the expected date format (YYYY-MM-DD)', () => {
+    const date: TDateISO = '2025-01-28'; // YYYY-MM-DD
+    expect(isDateISO(date)).toBe(true);
+  });
+
+  it('should match the expected date-time format (YYYY-MM-DDTHH:mm:ss.sssZ)', () => {
+    const date: TDateISO = '2025-01-28T14:42:34.678Z'; // YYYY-MM-DDTHH:mm:ss.sssZ
+    expect(isDateISO(date)).toBe(true);
+  });
+
+  it('should match the correct format for `toISOString()` method', () => {
+    const date = new Date();
+    const isoString: TDateISO = date.toISOString();
+    expect(isDateISO(isoString)).toBe(true);
+  });
+
+  it('should return a string in the correct format from `toISOString()`', () => {
+    const date = new Date();
+    const isoString = date.toISOString();
+    // Check if the ISO string matches either of the two formats
+    expect(isoString).toMatch(/^\d{4}-\d{2}-\d{2}(T\d{2}:\d{2}:\d{2}\.\d{3}Z)?$/);
+  });
+});
diff --git a/frontend/tests/Mock.test.tsx b/frontend/tests/Mock.test.tsx
index 554183e..9d0a152 100644
--- a/frontend/tests/Mock.test.tsx
+++ b/frontend/tests/Mock.test.tsx
@@ -48,7 +48,7 @@ describe("Account component", () => {
 
     // Act 1
     render(
-      <MemoryRouter>
+    <MemoryRouter>
       <AuthProvider>
         <Account />
       </AuthProvider>