diff --git a/docs/ERROR_CODE_REGISTRY.md b/docs/ERROR_CODE_REGISTRY.md
index f0fabd1..1b827d6 100644
--- a/docs/ERROR_CODE_REGISTRY.md
+++ b/docs/ERROR_CODE_REGISTRY.md
@@ -284,6 +284,55 @@ Error codes are defined in `src/constants/error.constants.ts` and should be trea
 }
 ```
 
+---
+### TOKEN_EXPIRY_TAMPERED
+
+**HTTP Status:** 401 Unauthorized
+
+**Meaning:** JWT token expiry has been tampered with. The token's `exp` claim does not match the expected `iat + ttlSeconds`.
+
+**When to use:**
+
+- Token's `exp` is greater than `iat + ttlSeconds` (expiry was extended beyond the valid window)
+- Token's `exp` is less than `iat + ttlSeconds` (expiry was shortened, token revoked early)
+
+**Example:**
+
+```json
+{
+   "success": false,
+   "error": {
+      "code": "TOKEN_EXPIRY_TAMPERED",
+      "message": "Token expiry has been tampered with"
+   }
+}
+```
+
+---
+
+### MISSING_IAT
+
+**HTTP Status:** 401 Unauthorized
+
+**Meaning:** JWT token is missing the `iat` (issued-at) claim. The token cannot have its expiry validated without an issuance timestamp.
+
+**When to use:**
+
+- Token does not include an `iat` claim
+- Token payload is decoded but `iat` is undefined
+
+**Example:**
+
+```json
+{
+   "success": false,
+   "error": {
+      "code": "MISSING_IAT",
+      "message": "Token is missing the issued-at (iat) claim"
+   }
+}
+```
+
 ---
 
 ## Adding New Error Codes
diff --git a/src/constants/error.constants.ts b/src/constants/error.constants.ts
index c207198..f50622c 100644
--- a/src/constants/error.constants.ts
+++ b/src/constants/error.constants.ts
@@ -15,6 +15,8 @@ export const ErrorCode = {
    JWT_ERROR: 'TOKEN_ERROR',
    INSUFFICIENT_BALANCE: 'insufficient_balance',
    NOT_A_CREATOR: 'not_a_creator',
+   TOKEN_EXPIRY_TAMPERED: 'token_expiry_tampered',
+   MISSING_IAT: 'missing_iat',
 } as const;
 
 export type ErrorCodeType = (typeof ErrorCode)[keyof typeof ErrorCode];
diff --git a/src/utils/token-expiry.validator.test.ts b/src/utils/token-expiry.validator.test.ts
new file mode 100644
index 0000000..1aaa69f
--- /dev/null
+++ b/src/utils/token-expiry.validator.test.ts
@@ -0,0 +1,53 @@
+import { validateTokenExpiry } from '../utils/token-expiry.validator';
+
+describe('validateTokenExpiry', () => {
+   it('accepts a token where exp === iat + ttl', () => {
+      const payload = {
+         iat: 1000,
+         exp: 1060,
+         ttlSeconds: 60,
+      };
+
+      const result = validateTokenExpiry(payload);
+
+      expect(result.valid).toBe(true);
+   });
+
+   it('rejects a token where exp > iat + ttl with token_expiry_tampered', () => {
+      const payload = {
+         iat: 1000,
+         exp: 1100,
+         ttlSeconds: 60,
+      };
+
+      const result = validateTokenExpiry(payload);
+
+      expect(result.valid).toBe(false);
+      expect(result.code).toBe('token_expiry_tampered');
+   });
+
+   it('rejects a token where exp < iat + ttl with token_expiry_tampered', () => {
+      const payload = {
+         iat: 1000,
+         exp: 1050,
+         ttlSeconds: 60,
+      };
+
+      const result = validateTokenExpiry(payload);
+
+      expect(result.valid).toBe(false);
+      expect(result.code).toBe('token_expiry_tampered');
+   });
+
+   it('rejects a token with no iat claim with missing_iat', () => {
+      const payload = {
+         exp: 1100,
+         ttlSeconds: 60,
+      };
+
+      const result = validateTokenExpiry(payload);
+
+      expect(result.valid).toBe(false);
+      expect(result.code).toBe('missing_iat');
+   });
+});
\ No newline at end of file
diff --git a/src/utils/token-expiry.validator.ts b/src/utils/token-expiry.validator.ts
new file mode 100644
index 0000000..2b44242
--- /dev/null
+++ b/src/utils/token-expiry.validator.ts
@@ -0,0 +1,35 @@
+/**
+ * Token expiry validator.
+ *
+ * Computes the expected expiry as iat + ttlSeconds and validates that
+ * the token's exp claim matches. Returns specific error codes for
+ * tampered expiry or missing iat claim.
+ */
+export interface TokenExpiryValidationResult {
+   valid: boolean;
+   code?: string;
+}
+
+export function validateTokenExpiry(
+   payload: { iat?: number; exp: number; ttlSeconds: number }
+): TokenExpiryValidationResult {
+   if (payload.iat === undefined) {
+      return { valid: false, code: 'missing_iat' };
+   }
+
+   const expectedExp = payload.iat + payload.ttlSeconds;
+
+   if (payload.exp === expectedExp) {
+      return { valid: true };
+   }
+
+   if (payload.exp > expectedExp) {
+      return { valid: false, code: 'token_expiry_tampered' };
+   }
+
+   if (payload.exp < expectedExp) {
+      return { valid: false, code: 'token_expiry_tampered' };
+   }
+
+   return { valid: false };
+}
\ No newline at end of file