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
 
diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index 9a62359..70fa5ca 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -17,7 +17,6 @@
 import mock
 import pytest
 
-from google.auth import _helpers
 from google.auth import app_engine
 
 
@@ -134,7 +133,7 @@
         return_value=datetime.datetime.min)
     def test_refresh(self, utcnow, app_identity):
         token = 'token'
-        ttl = _helpers.CLOCK_SKEW_SECS + 100
+        ttl = 643942923
         app_identity.get_access_token.return_value = token, ttl
         credentials = app_engine.Credentials(scopes=['email'])
 
@@ -143,8 +142,8 @@
         app_identity.get_access_token.assert_called_with(
             credentials.scopes, credentials._service_account_id)
         assert credentials.token == token
-        assert credentials.expiry == (
-            utcnow() + datetime.timedelta(seconds=ttl))
+        assert credentials.expiry == datetime.datetime(
+            1990, 5, 29, 1, 2, 3)
         assert credentials.valid
         assert not credentials.expired