diff --git a/jupyterhub/apihandlers/auth.py b/jupyterhub/apihandlers/auth.py
index 839fcc4a..e5b55665 100644
--- a/jupyterhub/apihandlers/auth.py
+++ b/jupyterhub/apihandlers/auth.py
@@ -8,7 +8,7 @@ import json
 from urllib.parse import quote
 
 from oauth2.web.tornado import OAuth2Handler
-from tornado import web, gen
+from tornado import web
 
 from .. import orm
 from ..user import User
diff --git a/jupyterhub/apihandlers/base.py b/jupyterhub/apihandlers/base.py
index 8223f35c..329d6eb7 100644
--- a/jupyterhub/apihandlers/base.py
+++ b/jupyterhub/apihandlers/base.py
@@ -10,7 +10,7 @@ from tornado import web
 
 from .. import orm
 from ..handlers import BaseHandler
-from ..utils import url_path_join
+from ..utils import isoformat, url_path_join
 
 class APIHandler(BaseHandler):
 
@@ -103,7 +103,7 @@ class APIHandler(BaseHandler):
         last_activity = user.last_activity
         # don't call isoformat if last_activity is None
         if last_activity:
-            last_activity = last_activity.isoformat()
+            last_activity = isoformat(last_activity)
 
         model = {
             'kind': 'user',
@@ -122,7 +122,7 @@ class APIHandler(BaseHandler):
             for name, spawner in user.spawners.items():
                 last_activity = spawner.orm_spawner.last_activity
                 if last_activity:
-                    last_activity = last_activity.isoformat()
+                    last_activity = isoformat(last_activity)
                 if spawner.ready:
                     servers[name] = s = {
                         'name': name,
diff --git a/jupyterhub/app.py b/jupyterhub/app.py
index 1b5ce1e0..31ce2fb1 100644
--- a/jupyterhub/app.py
+++ b/jupyterhub/app.py
@@ -8,7 +8,7 @@ import asyncio
 import atexit
 import binascii
 from concurrent.futures import ThreadPoolExecutor
-from datetime import datetime
+from datetime import datetime, timezone
 from getpass import getuser
 import logging
 from operator import itemgetter
@@ -22,8 +22,8 @@ from urllib.parse import urlparse
 if sys.version_info[:2] < (3, 3):
     raise ValueError("Python < 3.3 not supported: %s" % sys.version)
 
+from dateutil.parser import parse as parse_date
 from jinja2 import Environment, FileSystemLoader, PrefixLoader, ChoiceLoader
-
 from sqlalchemy.exc import OperationalError
 
 from tornado.httpclient import AsyncHTTPClient
@@ -50,7 +50,7 @@ from .services.service import Service
 
 from . import crypto
 from . import dbutil, orm
-from .user import User, UserDict
+from .user import UserDict
 from .oauth.store import make_provider
 from ._data import DATA_FILES_PATH
 from .log import CoroutineLogFormatter, log_request
@@ -59,7 +59,6 @@ from .traitlets import URLPrefix, Command
 from .utils import (
     maybe_future,
     url_path_join,
-    ISO8601_ms, ISO8601_s,
     print_stacks, print_ps_info,
 )
 # classes for config
@@ -1581,10 +1580,11 @@ class JupyterHub(Application):
             if spawner is None:
                 self.log.warning("Found no spawner for route: %s", route)
                 continue
-            try:
-                dt = datetime.strptime(route_data['last_activity'], ISO8601_ms)
-            except Exception:
-                dt = datetime.strptime(route_data['last_activity'], ISO8601_s)
+            dt = parse_date(route_data['last_activity'])
+            if dt.tzinfo:
+                # strip timezone info to naïve UTC datetime
+                dt = dt.astimezone(timezone.utc).replace(tzinfo=None)
+
             if user.last_activity:
                 user.last_activity = max(user.last_activity, dt)
             else:
diff --git a/jupyterhub/handlers/base.py b/jupyterhub/handlers/base.py
index b5713766..fdd01e86 100644
--- a/jupyterhub/handlers/base.py
+++ b/jupyterhub/handlers/base.py
@@ -26,7 +26,7 @@ from ..spawner import LocalProcessSpawner
 from ..utils import maybe_future, url_path_join
 from ..metrics import (
     SERVER_SPAWN_DURATION_SECONDS, ServerSpawnStatus,
-    PROXY_ADD_DURATION_SECONDS, ProxyAddStatus
+    PROXY_ADD_DURATION_SECONDS, ProxyAddStatus,
 )
 
 # pattern for the authentication token header
diff --git a/jupyterhub/user.py b/jupyterhub/user.py
index 598047cf..681ba8ce 100644
--- a/jupyterhub/user.py
+++ b/jupyterhub/user.py
@@ -10,7 +10,6 @@ from oauth2.error import ClientNotFoundError
 from sqlalchemy import inspect
 from tornado import gen
 from tornado.log import app_log
-from traitlets import HasTraits, Any, Dict, default
 
 from .utils import maybe_future, url_path_join
 
diff --git a/jupyterhub/utils.py b/jupyterhub/utils.py
index 21cb7ab6..6545ba6d 100644
--- a/jupyterhub/utils.py
+++ b/jupyterhub/utils.py
@@ -6,6 +6,7 @@
 import asyncio
 from binascii import b2a_hex
 import concurrent.futures
+from datetime import datetime, timezone
 import random
 import errno
 import hashlib
@@ -15,7 +16,6 @@ import os
 import socket
 import sys
 import threading
-from threading import Thread
 import uuid
 import warnings
 
@@ -39,6 +39,16 @@ ISO8601_ms = '%Y-%m-%dT%H:%M:%S.%fZ'
 ISO8601_s = '%Y-%m-%dT%H:%M:%SZ'
 
 
+def isoformat(dt):
+    """Render a datetime object as an ISO 8601 UTC timestamp
+
+    Naïve datetime objects are assumed to be UTC
+    """
+    if dt.tzinfo:
+        dt = dt.astimezone(timezone.utc).replace(tzinfo=None)
+    return dt.isoformat() + 'Z'
+
+
 def can_connect(ip, port):
     """Check if we can connect to an ip:port.
 
diff --git a/requirements.txt b/requirements.txt
index 7a71f61a..a3a47065 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,6 +4,7 @@ tornado>=4.1
 jinja2
 pamela
 python-oauth2>=1.0
+python-dateutil
 SQLAlchemy>=1.1
 requests
 prometheus_client>=0.0.21