diff --git a/lib/matplotlib/axes.py b/lib/matplotlib/axes.py
@@ -2716,18 +2716,20 @@ def set_yticklabels(self, labels, fontdict=None, minor=False, **kwargs):
     def xaxis_date(self, tz=None):
         """Sets up x-axis ticks and labels that treat the x data as dates.
 
-        *tz* is the time zone to use in labeling dates.  Defaults to rc value.
+        *tz* is a timezone string or :class:`tzinfo` instance.
+        Defaults to rc value.
         """
         # should be enough to inform the unit conversion interface
-        # dates are comng in
-        self.xaxis.axis_date()
+        # dates are coming in
+        self.xaxis.axis_date(tz)
 
     def yaxis_date(self, tz=None):
         """Sets up y-axis ticks and labels that treat the y data as dates.
 
-        *tz* is the time zone to use in labeling dates.  Defaults to rc value.
+        *tz* is a timezone string or :class:`tzinfo` instance.
+        Defaults to rc value.
         """
-        self.yaxis.axis_date()
+        self.yaxis.axis_date(tz)
 
     def format_xdata(self, x):
         """
@@ -3845,7 +3847,7 @@ def plot_date(self, x, y, fmt='bo', tz=None, xdate=True, ydate=False,
         *fmt*: string
            The plot format string.
 
-        *tz*: [ None | timezone string ]
+        *tz*: [ None | timezone string | :class:`tzinfo` instance]
            The time zone to use in labeling dates. If *None*, defaults to rc
            value.
 

diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -1218,21 +1218,21 @@ def grid(self, b=None, which='major', **kwargs):
     def update_units(self, data):
         """
         introspect *data* for units converter and update the
-        axis.converter instance if necessary. Return *True* is *data* is
-        registered for unit conversion
+        axis.converter instance if necessary. Return *True*
+        if *data* is registered for unit conversion.
         """
 
         converter = munits.registry.get_converter(data)
-        if converter is None: return False
+        if converter is None:
+            return False
 
         neednew = self.converter!=converter
         self.converter = converter
         default = self.converter.default_units(data, self)
-        #print 'update units: default="%s", units=%s"'%(default, self.units)
+        #print 'update units: default=%s, units=%s'%(default, self.units)
         if default is not None and self.units is None:
             self.set_units(default)
 
-
         if neednew:
             self._update_axisinfo()
         return True
@@ -1453,14 +1453,21 @@ def zoom(self, direction):
         self.major.locator.zoom(direction)
 
 
-    def axis_date(self):
+    def axis_date(self, tz=None):
         """
         Sets up x-axis ticks and labels that treat the x data as dates.
+        *tz* is a :class:`tzinfo` instance or a timezone string.
+        This timezone is used to create date labels.
         """
+        # By providing a sample datetime instance with the desired
+        # timezone, the registered converter can be selected,
+        # and the "units" attribute, which is the timezone, can
+        # be set.
         import datetime
-        # should be enough to inform the unit conversion interface
-        # dates are comng in
-        self.update_units(datetime.date(2009,1,1))
+        if isinstance(tz, (str, unicode)):
+            import pytz
+            tz = pytz.timezone(tz)
+        self.update_units(datetime.datetime(2009,1,1,0,0,0,0,tz))
 
 
 class XAxis(Axis):

diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -1104,15 +1104,26 @@ def weeks(w):
 
 
 class DateConverter(units.ConversionInterface):
-    """The units are equivalent to the timezone."""
+    """
+    Converter for datetime.date and datetime.datetime data,
+    or for date/time data represented as it would be converted
+    by :func:`date2num`.
+
+    The 'unit' tag for such data is None or a tzinfo instance.
+    """
 
     @staticmethod
     def axisinfo(unit, axis):
-        'return the unit AxisInfo'
-        # make sure that the axis does not start at 0
+        """
+        Return the :class:`~matplotlib.units.AxisInfo` for *unit*.
+
+        *unit* is a tzinfo instance or None.
+        The *axis* argument is required but not used.
+        """
+        tz = unit
 
-        majloc = AutoDateLocator(tz=unit)
-        majfmt = AutoDateFormatter(majloc, tz=unit)
+        majloc = AutoDateLocator(tz=tz)
+        majfmt = AutoDateFormatter(majloc, tz=tz)
         datemin = datetime.date(2000, 1, 1)
         datemax = datetime.date(2010, 1, 1)
 
@@ -1121,12 +1132,28 @@ def axisinfo(unit, axis):
 
     @staticmethod
     def convert(value, unit, axis):
-        if units.ConversionInterface.is_numlike(value): return value
+        """
+        If *value* is not already a number or sequence of numbers,
+        convert it with :func:`date2num`.
+
+        The *unit* and *axis* arguments are not used.
+        """
+        if units.ConversionInterface.is_numlike(value):
+            return value
         return date2num(value)
 
     @staticmethod
     def default_units(x, axis):
-        'Return the default unit for *x* or None'
+        'Return the tzinfo instance of *x* or of its first element, or None'
+        try:
+            x = x[0]
+        except (TypeError, IndexError):
+            pass
+
+        try:
+            return x.tzinfo
+        except AttributeError:
+            pass
         return None
 
 

diff --git a/lib/matplotlib/units.py b/lib/matplotlib/units.py
@@ -7,7 +7,7 @@
 are unit aware.  We don't assume any particular units implementation,
 rather a units implementation must provide a ConversionInterface, and
 the register with the Registry converter dictionary.  For example,
-here is a complete implementation which support plotting with native
+here is a complete implementation which supports plotting with native
 datetime objects::
 
 
@@ -48,7 +48,7 @@ def default_units(x, axis):
 class AxisInfo:
     'information to support default axis labeling and tick labeling, and default limits'
     def __init__(self, majloc=None, minloc=None,
-                 majfmt=None, minfmt=None, label=None, 
+                 majfmt=None, minfmt=None, label=None,
                  default_limits=None):
         """
         majloc and minloc: TickLocators for the major and minor ticks