add timezone info to `last_modified` in notebook managers

now Firefox won't get confused, turning utc timestamps into local times.

closes  #3396

IPython/html/services/notebooks/azurenbmanager.py

@@ -26,7 +26,7 b' from tornado import web'
 from .nbmanager import NotebookManager
 from IPython.nbformat import current
 from IPython.utils.traitlets import Unicode, Instance
-
+from IPython.utils import tz
 
 #-----------------------------------------------------------------------------
 # Classes
@@ -98,7 +98,7 b' class AzureNotebookManager(NotebookManager):'
             raise web.HTTPError(500, u'Unreadable JSON notebook.')
         # Todo: The last modified should actually be saved in the notebook document.
         # We are just using the current datetime until that is implemented.
-        last_modified = datetime.datetime.utcnow()
+        last_modified = tz.utcnow()
         return last_modified, nb
 
     def write_notebook_object(self, nb, notebook_id=None):

IPython/html/services/notebooks/filenbmanager.py

@@ -28,6 +28,7 b' from tornado import web'
 from .nbmanager import NotebookManager
 from IPython.nbformat import current
 from IPython.utils.traitlets import Unicode, Dict, Bool, TraitError
+from IPython.utils import tz
 
 #-----------------------------------------------------------------------------
 # Classes
@@ -139,7 +140,7 b' class FileNotebookManager(NotebookManager):'
     def read_notebook_object_from_path(self, path):
         """read a notebook object from a path"""
         info = os.stat(path)
-        last_modified = datetime.datetime.utcfromtimestamp(info.st_mtime)
+        last_modified = tz.utcfromtimestamp(info.st_mtime)
         with open(path,'r') as f:
             s = f.read()
             try:
@@ -281,7 +282,7 b' class FileNotebookManager(NotebookManager):'
         """construct the info dict for a given checkpoint"""
         path = self.get_checkpoint_path(notebook_id, checkpoint_id)
         stats = os.stat(path)
-        last_modified = datetime.datetime.utcfromtimestamp(stats.st_mtime)
+        last_modified = tz.utcfromtimestamp(stats.st_mtime)
         info = dict(
             checkpoint_id = checkpoint_id,
             last_modified = last_modified,