diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5565688..628f1e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Add `expirationDate` (ISO 8601 string, or `None` when the license never expires) to the `activate_key` and `floating_checkout` response types. On checkout it is separate from `expiresAt`, the seat lease. Additive; no other fields changed.
+- Document reading the expiry, computing days left, and storing the last value in the README.
+
 ## 2.5.0 - 2026-09-09
 
 - Synchronize licensing, floating-session, offline-signing, error, and customer contracts with the current API.
diff --git a/README.md b/README.md
index e1d9c3f..76a2223 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,30 @@ if result and 'key' in result:
     print(f"Created Key: {result['key']}")
 ```
 
+## License Expiry
+
+`activate_key` and `floating_checkout` return `expirationDate`: the license expiry as an ISO 8601 string (e.g. `"2027-03-01T00:00:00.000Z"`), or `None` when the license never expires. On `floating_checkout`, `expiresAt` is the seat lease, not the license expiry.
+
+```python
+from datetime import datetime, timezone
+import math
+
+result = sdk.activate_key({'productId': product_id, 'licenseKey': license_key, 'hostId': host_id})
+
+expiration = result.get('expirationDate')
+if expiration is None:
+    print('License never expires')
+else:
+    expires_at = datetime.fromisoformat(expiration.replace('Z', '+00:00'))
+    seconds_left = (expires_at - datetime.now(timezone.utc)).total_seconds()
+    days_left = max(0, math.ceil(seconds_left / 86400))
+    print(f'License expires {expires_at:%Y-%m-%d} ({days_left} days left)')
+
+# Store the last value you received (e.g. in your app's settings file).
+```
+
+Store the last `expirationDate` you received. An expired license fails activation with the same generic invalid-key error as a wrong key, so the stored date is how your app can tell the user the license expired on that date instead of calling the key invalid.
+
 ## Machine Identity
 Keymint provides utilities to uniquely identify machines for node-locking:
 
diff --git a/keymint/__init__.py b/keymint/__init__.py
index 8e8e94a..e6b9a31 100644
--- a/keymint/__init__.py
+++ b/keymint/__init__.py
@@ -79,7 +79,10 @@ def activate_key(self, params: ActivateKeyParams, idempotency_key: str | None =
         
         :param params: Activation parameters including productId, licenseKey, and optional hostId.
         :param idempotency_key: Optional unique identifier to ensure request idempotency.
-        :return: Activation success message and licensee information.
+        :return: Activation success message and licensee information. Includes
+            ``expirationDate``: the license expiry as an ISO 8601 string, or ``None``
+            when the license never expires. An expired license fails activation with
+            the same generic invalid-key error, so store the last value you received.
         """
         return self._handle_request('POST', '/key/activate', params, idempotency_key=idempotency_key)
 
@@ -98,6 +101,8 @@ def floating_checkout(self, params: FloatingCheckoutParams, idempotency_key: str
         :param params: Parameters for checking out the license.
         :param idempotency_key: Optional unique identifier to ensure request idempotency.
         :returns: The checkout response containing sessionId and sessionSecret.
+            ``expiresAt`` is the seat lease; ``expirationDate`` is the license expiry
+            as an ISO 8601 string, or ``None`` when the license never expires.
         """
         return self._handle_request('POST', '/key/checkout', params, idempotency_key=idempotency_key)
 
diff --git a/keymint/types.py b/keymint/types.py
index eb1df5f..f3d0e9d 100644
--- a/keymint/types.py
+++ b/keymint/types.py
@@ -59,6 +59,9 @@ class ActivateKeyResponse(TypedDict):
     versionId: str | None
     version: dict[str, Any] | None
     allowedHosts: list[str] | None
+    # License expiry as an ISO 8601 string (e.g. "2027-03-01T00:00:00.000Z"),
+    # or None when the license never expires.
+    expirationDate: str | None
 
 class DeactivateKeyParams(TypedDict):
     productId: str
@@ -223,9 +226,12 @@ class FloatingCheckoutResponse(TypedDict):
     sessionId: str
     sessionSecret: str
     nextNonce: str
-    expiresAt: str
+    expiresAt: str  # Floating seat lease expiry (not the license expiry)
     heartbeatInterval: int
     metadata: dict[str, Any] | None
+    # License expiry as an ISO 8601 string, or None when the license never
+    # expires. Separate from ``expiresAt``, which is the seat lease.
+    expirationDate: str | None
     currentSessions: int | None
     maxSessions: int | None
     licenseeName: str | None
diff --git a/tests/test_contracts.py b/tests/test_contracts.py
index 5b6f403..2b6275d 100644
--- a/tests/test_contracts.py
+++ b/tests/test_contracts.py
@@ -50,3 +50,36 @@ def test_nested_error_preserves_server_message(mock_post):
 
     with pytest.raises(KeyMintApiError, match="Customer email already exists in team"):
         KeyMint("admin_test").create_key({"productId": "product_123"})
+
+
+@pytest.mark.parametrize("expiration_date", ["2027-03-01T00:00:00.000Z", None])
+@patch("keymint.requests.post")
+def test_activate_key_returns_expiration_date(mock_post, expiration_date):
+    mock_post.return_value = response({
+        "code": 0, "message": "License valid", "expirationDate": expiration_date,
+    })
+
+    result = KeyMint("client_test").activate_key({"productId": "product_123", "licenseKey": "license_123"})
+
+    assert mock_post.call_args.args[0].endswith("/key/activate")
+    assert "expirationDate" in result
+    assert result["expirationDate"] == expiration_date
+
+
+@pytest.mark.parametrize("expiration_date", ["2027-03-01T00:00:00.000Z", None])
+@patch("keymint.requests.post")
+def test_floating_checkout_returns_expiration_date_separate_from_lease(mock_post, expiration_date):
+    mock_post.return_value = response({
+        "code": 0, "message": "Checked out", "sessionId": "session_123",
+        "sessionSecret": "secret_123", "nextNonce": "nonce_123",
+        "expiresAt": "2026-10-26T12:05:00.000Z", "heartbeatInterval": 60,
+        "metadata": None, "expirationDate": expiration_date,
+    })
+
+    result = KeyMint("client_test").floating_checkout({
+        "productId": "product_123", "licenseKey": "license_123", "hostId": "host_123",
+    })
+
+    assert mock_post.call_args.args[0].endswith("/key/checkout")
+    assert result["expirationDate"] == expiration_date
+    assert result["expiresAt"] == "2026-10-26T12:05:00.000Z"