diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index 6dc8712..fa13f8e 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -114,7 +114,7 @@
         # pylint: disable=unused-argument
         token, ttl = app_identity.get_access_token(
             self._scopes, self._service_account_id)
-        expiry = _helpers.utcnow() + datetime.timedelta(seconds=ttl)
+        expiry = datetime.datetime.utcfromtimestamp(ttl)
 
         self.token, self.expiry = token, expiry