allow datestamps to exclude microseconds

datetime.isoformat() excludes microseconds when the value is zero,
causing a 1/1000000 failure on most platforms, and 1/1000 on Windows due to millisecond datetime resolution.

IPython/utils/jsonutil.py

@@ -34,7 +34,7 b" next_attr_name = '__next__' if py3compat.PY3 else 'next'"
 
 # timestamp formats
 ISO8601 = "%Y-%m-%dT%H:%M:%S.%f"
-ISO8601_PAT=re.compile(r"^(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{1,6})Z?([\+\-]\d{2}:?\d{2})?$")
+ISO8601_PAT=re.compile(r"^(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2})(\.\d{1,6})?Z?([\+\-]\d{2}:?\d{2})?$")
 
 #-----------------------------------------------------------------------------
 # Classes and functions
@@ -75,7 +75,10 b' def parse_date(s):'
     if m:
         # FIXME: add actual timezone support
         # this just drops the timezone info
-        notz = m.groups()[0]
+        notz, ms, tz = m.groups()
+        if not ms:
+            ms = '.0'
+        notz = notz + ms
         return datetime.strptime(notz, ISO8601)
     return s