diff --git a/IPython/parallel/client/client.py b/IPython/parallel/client/client.py
index 6281df9..27e5d45 100644
--- a/IPython/parallel/client/client.py
+++ b/IPython/parallel/client/client.py
@@ -37,7 +37,7 @@ from IPython.core.profiledir import ProfileDir, ProfileDirError
 
 from IPython.utils.capture import RichOutput
 from IPython.utils.coloransi import TermColors
-from IPython.utils.jsonutil import rekey, extract_dates
+from IPython.utils.jsonutil import rekey, extract_dates, parse_date
 from IPython.utils.localinterfaces import localhost, is_local_ip
 from IPython.utils.path import get_ipython_dir
 from IPython.utils.py3compat import cast_bytes, string_types, xrange, iteritems
@@ -675,7 +675,7 @@ class Client(HasTraits):
         if 'date' in parent:
             md['submitted'] = parent['date']
         if 'started' in msg_meta:
-            md['started'] = extract_dates(msg_meta['started'])
+            md['started'] = parse_date(msg_meta['started'])
         if 'date' in header:
             md['completed'] = header['date']
         return md
@@ -1580,7 +1580,7 @@ class Client(HasTraits):
                 )
                 md.update(self._extract_metadata(md_msg))
                 if rec.get('received'):
-                    md['received'] = extract_dates(rec['received'])
+                    md['received'] = parse_date(rec['received'])
                 md.update(iodict)
                 
                 if rcontent['status'] == 'ok':
@@ -1843,12 +1843,12 @@ class Client(HasTraits):
         has_rbufs = result_buffer_lens is not None
         for i,rec in enumerate(records):
             # unpack datetime objects
-            for dtkey in ('header', 'result_header',
-                        'submitted', 'started',
-                        'completed', 'received',
-                    ):
+            for hkey in ('header', 'result_header'):
+                if hkey in rec:
+                    rec[hkey] = extract_dates(rec[hkey])
+            for dtkey in ('submitted', 'started', 'completed', 'received'):
                 if dtkey in rec:
-                    rec[dtkey] = extract_dates(rec[dtkey])
+                    rec[dtkey] = parse_date(rec[dtkey])
             # relink buffers
             if has_bufs:
                 blen = buffer_lens[i]
diff --git a/IPython/utils/jsonutil.py b/IPython/utils/jsonutil.py
index ee93a06..565b1bc 100644
--- a/IPython/utils/jsonutil.py
+++ b/IPython/utils/jsonutil.py
@@ -62,22 +62,34 @@ def rekey(dikt):
             dikt[nk] = dikt.pop(k)
     return dikt
 
+def parse_date(s):
+    """parse an ISO8601 date string
+    
+    If it is None or not a valid ISO8601 timestamp,
+    it will be returned unmodified.
+    Otherwise, it will return a datetime object.
+    """
+    if s is None:
+        return s
+    m = ISO8601_PAT.match(s)
+    if m:
+        # FIXME: add actual timezone support
+        # this just drops the timezone info
+        notz = m.groups()[0]
+        return datetime.strptime(notz, ISO8601)
+    return s
 
 def extract_dates(obj):
     """extract ISO8601 dates from unpacked JSON"""
     if isinstance(obj, dict):
-        obj = dict(obj) # don't clobber
+        new_obj = {} # don't clobber
         for k,v in iteritems(obj):
-            obj[k] = extract_dates(v)
+            new_obj[k] = extract_dates(v)
+        obj = new_obj
     elif isinstance(obj, (list, tuple)):
         obj = [ extract_dates(o) for o in obj ]
     elif isinstance(obj, string_types):
-        m = ISO8601_PAT.match(obj)
-        if m:
-            # FIXME: add actual timezone support
-            # this just drops the timezone info
-            notz = m.groups()[0]
-            obj = datetime.strptime(notz, ISO8601)
+        obj = parse_date(obj)
     return obj
 
 def squash_dates(obj):