diff --git a/tests/test_client.py b/tests/test_client.py
index e334054..1b4c207 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -161,7 +161,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 {},
-                3600 * 1000  # 1 hour
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/file.jpg?auth_key=test-key&exp=1732550672867&sig=sha256%3Ad994b8a737db1c43d6e04a07018dc33e8e28b23b27854bd6383d828a212cfffb'
@@ -176,7 +176,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 {},
-                3600 * 1000
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/input.jpg?auth_key=test-key&exp=1732550672867&sig=sha256%3A75991f02828d194792c9c99f8fea65761bcc4c62dbb287a84f642033128297c0'
@@ -192,7 +192,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 params['url_params'],
-                3600 * 1000
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/file.jpg?auth_key=test-key&exp=1732550672867&width=100&sig=sha256%3Ae5271d8fb6482d9351ebe4285b6fc75539c4d311ff125c4d76d690ad71c258ef'
@@ -207,7 +207,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 params['url_params'],
-                3600 * 1000
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/file.jpg?auth_key=test-key&exp=1732550672867&height=200&width=&sig=sha256%3A1a26733c859f070bc3d83eb3174650d7a0155642e44a5ac448a43bc728bc0f85'
@@ -222,7 +222,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 params['url_params'],
-                3600 * 1000
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/file.jpg?auth_key=test-key&exp=1732550672867&height=200&sig=sha256%3Adb740ebdfad6e766ebf6516ed5ff6543174709f8916a254f8d069c1701cef517'
@@ -237,7 +237,7 @@ def test_get_signed_smart_cdn_url(self):
                 params['template'],
                 params['input'],
                 params['url_params'],
-                3600 * 1000
+                expires_at_ms=self.expire_at_ms
             )
 
         expected_url = 'https://workspace.tlcdn.com/template/file.jpg?auth_key=test-key&exp=1732550672867&width=&sig=sha256%3A840426f9ac72dde02fd080f09b2304d659fdd41e630b1036927ec1336c312e9d'
diff --git a/transloadit/client.py b/transloadit/client.py
index 77f8125..4d659d2 100644
--- a/transloadit/client.py
+++ b/transloadit/client.py
@@ -179,7 +179,7 @@ def get_signed_smart_cdn_url(
         template: str,
         input: str,
         url_params: Optional[dict[str, Union[str, int, float, bool, List[Union[str, int, float, bool]], None]]] = None,
-        expires_in: Optional[int] = 60 * 60 * 1000 # 1 hour
+        expires_at_ms: Optional[int] = None
     ) -> str:
         """
         Construct a signed Smart CDN URL.
@@ -190,7 +190,7 @@ def get_signed_smart_cdn_url(
             - template (str): Template slug or template ID
             - input (str): Input value that is provided as ${fields.input} in the template
             - url_params (Optional[dict]): Additional parameters for the URL query string. Values can be strings, numbers, booleans, arrays thereof, or None.
-            - expires_in (Optional[int]): Expiration time of signature in milliseconds. Defaults to 1 hour.
+            - expires_at_ms (Optional[int]): Timestamp in milliseconds since epoch when the signature is no longer valid. Defaults to 1 hour from now.
 
         :Returns:
             str: The signed Smart CDN URL
@@ -202,6 +202,8 @@ def get_signed_smart_cdn_url(
         template_slug = quote_plus(template)
         input_field = quote_plus(input)
 
+        expiry = expires_at_ms if expires_at_ms is not None else int(time.time() * 1000) + 60 * 60 * 1000  # 1 hour default
+
         params = []
         if url_params:
             for k, v in url_params.items():
@@ -215,7 +217,7 @@ def get_signed_smart_cdn_url(
                     raise ValueError(f"URL parameter values must be strings, numbers, booleans, arrays, or None. Got {type(v)} for {k}")
 
         params.append(("auth_key", self.auth_key))
-        params.append(("exp", str(int(time.time() * 1000) + expires_in)))
+        params.append(("exp", str(expiry)))
 
         # Sort params alphabetically by key
         sorted_params = sorted(params, key=lambda x: x[0])