diff --git a/notebook/services/contents/tz.py b/notebook/services/contents/tz.py
index b315d532d..1df39e51d 100644
--- a/notebook/services/contents/tz.py
+++ b/notebook/services/contents/tz.py
@@ -5,22 +5,11 @@ Timezone utilities
 Just UTC-awareness right now
 """
 
-#-----------------------------------------------------------------------------
-#  Copyright (C) 2013 The IPython Development Team
-#
-#  Distributed under the terms of the BSD License.  The full license is in
-#  the file COPYING, distributed as part of this software.
-#-----------------------------------------------------------------------------
-
-#-----------------------------------------------------------------------------
-# Imports
-#-----------------------------------------------------------------------------
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
 
 from datetime import tzinfo, timedelta, datetime
 
-#-----------------------------------------------------------------------------
-# Code
-#-----------------------------------------------------------------------------
 # constant for zero offset
 ZERO = timedelta(0)
 
@@ -44,3 +33,10 @@ def utc_aware(unaware):
 
 utcfromtimestamp = utc_aware(datetime.utcfromtimestamp)
 utcnow = utc_aware(datetime.utcnow)
+
+def isoformat(dt):
+    """Return iso-formatted timestamp
+    
+    Like .isoformat(), but uses Z for UTC instead of +00:00
+    """
+    return dt.isoformat().replace('+00:00', 'Z')
diff --git a/notebook/services/kernels/kernelmanager.py b/notebook/services/kernels/kernelmanager.py
index 87656e2af..e561d3983 100644
--- a/notebook/services/kernels/kernelmanager.py
+++ b/notebook/services/kernels/kernelmanager.py
@@ -17,7 +17,7 @@ from jupyter_client.multikernelmanager import MultiKernelManager
 from traitlets import Dict, List, Unicode, TraitError, default, validate
 
 from notebook.utils import to_os_path
-from notebook.services.contents.tz import utcnow
+from notebook.services.contents.tz import utcnow, isoformat
 from ipython_genutils.py3compat import getcwd
 
 
@@ -177,7 +177,7 @@ class MappingKernelManager(MultiKernelManager):
         model = {
             "id":kernel_id,
             "name": kernel.kernel_name,
-            "last_activity": kernel.last_activity,
+            "last_activity": isoformat(kernel.last_activity),
             "execution_state": kernel.execution_state,
             "connections": self._kernel_connections[kernel_id],
         }
diff --git a/notebook/services/sessions/tests/test_sessionmanager.py b/notebook/services/sessions/tests/test_sessionmanager.py
index 1e0202615..d8c8a28e3 100644
--- a/notebook/services/sessions/tests/test_sessionmanager.py
+++ b/notebook/services/sessions/tests/test_sessionmanager.py
@@ -9,14 +9,14 @@ from tornado.ioloop import IOLoop
 from ..sessionmanager import SessionManager
 from notebook.services.kernels.kernelmanager import MappingKernelManager
 from notebook.services.contents.manager import ContentsManager
-from notebook.services.contents.tz import utcnow
+from notebook.services.contents.tz import utcnow, isoformat
 
 class DummyKernel(object):
     def __init__(self, kernel_name='python'):
         self.kernel_name = kernel_name
 
 dummy_date = utcnow()
-dummy_date_s = dummy_date.isoformat()
+dummy_date_s = isoformat(dummy_date)
 
 class DummyMKM(MappingKernelManager):
     """MappingKernelManager interface that doesn't start kernels, for testing"""
@@ -74,7 +74,7 @@ class TestSessionManager(TestCase):
                     'kernel': {
                         'id': 'A',
                         'name': 'bar',
-                        'last_activity': dummy_date,
+                        'last_activity': dummy_date_s,
                         'execution_state': 'idle',
                     }}
         self.assertEqual(model, expected)
@@ -116,7 +116,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'A',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }, {
@@ -127,7 +127,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'B',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }, {
@@ -138,7 +138,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'C',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }
@@ -164,7 +164,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'B',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }
@@ -185,7 +185,7 @@ class TestSessionManager(TestCase):
                     'kernel': {
                         'id': 'A',
                         'name':'julia',
-                        'last_activity': dummy_date,
+                        'last_activity': dummy_date_s,
                         'execution_state': 'idle',
                     }
         }
@@ -216,7 +216,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'A',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }, {
@@ -227,7 +227,7 @@ class TestSessionManager(TestCase):
                 'kernel': {
                     'id': 'C',
                     'name':'python',
-                    'last_activity': dummy_date,
+                    'last_activity': dummy_date_s,
                     'execution_state': 'idle',
                 }
             }
diff --git a/notebook/services/sessions/tests/test_sessions_api.py b/notebook/services/sessions/tests/test_sessions_api.py
index dba1417d6..a947316df 100644
--- a/notebook/services/sessions/tests/test_sessions_api.py
+++ b/notebook/services/sessions/tests/test_sessions_api.py
@@ -225,6 +225,8 @@ class SessionAPITest(NotebookTestBase):
         r = self.request('GET', 'api/kernels')
         r.raise_for_status()
         kernel_list = r.json()
+        after['kernel'].pop('last_activity')
+        [ k.pop('last_activity') for k in kernel_list ]
         self.assertEqual(kernel_list, [after['kernel']])
 
     def test_modify_kernel_id(self):
@@ -248,4 +250,7 @@ class SessionAPITest(NotebookTestBase):
         r = self.request('GET', 'api/kernels')
         r.raise_for_status()
         kernel_list = r.json()
+
+        kernel.pop('last_activity')
+        [ k.pop('last_activity') for k in kernel_list ]
         self.assertEqual(kernel_list, [kernel])