diff --git a/Lib/datetime.py b/Lib/datetime.py
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1490,14 +1490,36 @@
         return datetime(year, month, day, hour, minute, second,
                           microsecond, tzinfo)
 
-    def astimezone(self, tz):
-        if not isinstance(tz, tzinfo):
-            raise TypeError("tz argument must be an instance of tzinfo")
-
+    def astimezone(self, tz=None):
         mytz = self.tzinfo
         if mytz is None:
             raise ValueError("astimezone() requires an aware datetime")
 
+        if tz is None:
+           ts = (self - _EPOCH) // timedelta(seconds=1)
+           localtm = _time.localtime(ts)
+           local = datetime(*localtm[:6])
+           try:
+               # Extract TZ data if available 
+               gmtoff = localtm.tm_gmtoff
+               zone = localtm.tm_zone
+           except AttributeError:
+               # Compute UTC offset and compare with the value implied
+               # by tm_isdst.  If the values match, use the zone name
+               # implied by tm_isdst.
+               delta = local - datetime(*_time.gmtime(ts)[:6])
+               dst = _time.daylight and localtm.tm_isdst > 0
+               gmtoff = _time.altzone if dst else _time.timezone
+               if delta == timedelta(seconds=-gmtoff):
+                   tz = timezone(delta, _time.tzname[dst])
+               else:
+                   tz = timezone(delta)
+           else:
+               tz = timezone(timedelta(seconds=-gmtoff), zone)
+
+        elif not isinstance(tz, tzinfo):
+            raise TypeError("tz argument must be an instance of tzinfo")
+
         if tz is mytz:
             return self