Fixed #17260 -- Added time zone aware aggregation and lookups.

Thanks Carl Meyer for the review.

Squashed commit of the following:

commit 4f290bdb60b7d8534abf4ca901bd0844612dcbda
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 21:21:30 2013 +0100

    Used '0:00' instead of 'UTC' which doesn't always exist in Oracle.

    Thanks Ian Kelly for the suggestion.

commit 01b6366f3ce67d57a58ca8f25e5be77911748638
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 13:38:43 2013 +0100

    Made tzname a parameter of datetime_extract/trunc_sql.

    This is required to work around a bug in Oracle.

commit 924a144ef8a80ba4daeeafbe9efaa826566e9d02
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 14:47:44 2013 +0100

    Added support for parameters in SELECT clauses.

commit b4351d2890cd1090d3ff2d203fe148937324c935
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 22:30:22 2013 +0100

    Documented backwards incompatibilities in the two previous commits.

commit 91ef84713c81bd455f559dacf790e586d08cacb9
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 09:42:31 2013 +0100

    Used QuerySet.datetimes for the admin's date_hierarchy.

commit 0d0de288a5210fa106cd4350961eb2006535cc5c
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 09:29:38 2013 +0100

    Used QuerySet.datetimes in date-based generic views.

commit 9c0859ff7c0b00734afe7fc15609d43d83215072
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:25 2013 +0100

    Implemented QuerySet.datetimes on Oracle.

commit 68ab511a4ffbd2b811bf5da174d47e4dd90f28fc
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:14 2013 +0100

    Implemented QuerySet.datetimes on MySQL.

commit 22d52681d347a8cdf568dc31ed032cbc61d049ef
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:42:29 2013 +0100

    Implemented QuerySet.datetimes on SQLite.

commit f6800fd04c93722b45f9236976389e0b2fe436f5
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:03 2013 +0100

    Implemented QuerySet.datetimes on PostgreSQL.

commit 0c829c23f4cf4d6804cadcc93032dd4c26b8c65e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:41:08 2013 +0100

    Added datetime-handling infrastructure in the ORM layers.

commit 104d82a7778cf3f0f5d03dfa53709c26df45daad
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 10:05:55 2013 +0100

    Updated null_queries tests to avoid clashing with the __second lookup.

commit c01bbb32358201b3ac8cb4291ef87b7612a2b8e6
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 23:07:41 2013 +0100

    Updated tests of .dates().

    Replaced .dates() by .datetimes() for DateTimeFields.
    Replaced dates with datetimes in the expected output for DateFields.

commit 50fb7a52462fecf0127b38e7f3df322aeb287c43
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:40:09 2013 +0100

    Updated and added tests for QuerySet.datetimes.

commit a8451a5004c437190e264667b1e6fb8acc3c1eeb
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 22:34:46 2013 +0100

    Documented the new time lookups and updated the date lookups.

commit 29413eab2bd1d5e004598900c0dadc0521bbf4d3
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 16:15:49 2013 +0100

    Documented QuerySet.datetimes and updated QuerySet.dates.

1 files changed, 136 insertions, 21 deletions

diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 171c2d3dcd..f77f87dd8e 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -550,14 +550,19 @@ dates
 .. method:: dates(field, kind, order='ASC')
 
 Returns a ``DateQuerySet`` — a ``QuerySet`` that evaluates to a list of
-``datetime.datetime`` objects representing all available dates of a particular
-kind within the contents of the ``QuerySet``.
+:class:`datetime.date` objects representing all available dates of a
+particular kind within the contents of the ``QuerySet``.
 
-``field`` should be the name of a ``DateField`` or ``DateTimeField`` of your
-model.
+.. versionchanged:: 1.6
+    ``dates`` used to return a list of :class:`datetime.datetime` objects.
+
+``field`` should be the name of a ``DateField`` of your model.
+
+.. versionchanged:: 1.6
+    ``dates`` used to accept operating on a ``DateTimeField``.
 
 ``kind`` should be either ``"year"``, ``"month"`` or ``"day"``. Each
-``datetime.datetime`` object in the result list is "truncated" to the given
+``datetime.date`` object in the result list is "truncated" to the given
 ``type``.
 
 * ``"year"`` returns a list of all distinct year values for the field.
@@ -572,21 +577,60 @@ model.
 Examples::
 
     >>> Entry.objects.dates('pub_date', 'year')
-    [datetime.datetime(2005, 1, 1)]
+    [datetime.date(2005, 1, 1)]
     >>> Entry.objects.dates('pub_date', 'month')
-    [datetime.datetime(2005, 2, 1), datetime.datetime(2005, 3, 1)]
+    [datetime.date(2005, 2, 1), datetime.date(2005, 3, 1)]
     >>> Entry.objects.dates('pub_date', 'day')
-    [datetime.datetime(2005, 2, 20), datetime.datetime(2005, 3, 20)]
+    [datetime.date(2005, 2, 20), datetime.date(2005, 3, 20)]
     >>> Entry.objects.dates('pub_date', 'day', order='DESC')
-    [datetime.datetime(2005, 3, 20), datetime.datetime(2005, 2, 20)]
+    [datetime.date(2005, 3, 20), datetime.date(2005, 2, 20)]
     >>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
-    [datetime.datetime(2005, 3, 20)]
+    [datetime.date(2005, 3, 20)]
 
-.. warning::
+datetimes
+~~~~~~~~~
+
+.. versionadded:: 1.6
+
+.. method:: datetimes(field, kind, order='ASC', tzinfo=None)
+
+Returns a ``DateTimeQuerySet`` — a ``QuerySet`` that evaluates to a list of
+:class:`datetime.datetime` objects representing all available dates of a
+particular kind within the contents of the ``QuerySet``.
+
+``field`` should be the name of a ``DateTimeField`` of your model.
+
+``kind`` should be either ``"year"``, ``"month"``, ``"day"``, ``"hour"``,
+``"minute"`` or ``"second"``. Each ``datetime.datetime`` object in the result
+list is "truncated" to the given ``type``.
+
+``order``, which defaults to ``'ASC'``, should be either ``'ASC'`` or
+``'DESC'``. This specifies how to order the results.
+
+``tzinfo`` defines the time zone to which datetimes are converted prior to
+truncation. Indeed, a given datetime has different representations depending
+on the time zone in use. This parameter must be a :class:`datetime.tzinfo`
+object. If it's ``None``, Django uses the :ref:`current time zone
+<default-current-time-zone>`. It has no effect when :setting:`USE_TZ` is
+``False``.
+
+.. _database-time-zone-definitions:
+
+.. note::
 
-    When :doc:`time zone support </topics/i18n/timezones>` is enabled, Django
-    uses UTC in the database connection, which means the aggregation is
-    performed in UTC. This is a known limitation of the current implementation.
+    This function performs time zone conversions directly in the database.
+    As a consequence, your database must be able to interpret the value of
+    ``tzinfo.tzname(None)``. This translates into the following requirements:
+
+    - SQLite: install pytz_ — conversions are actually performed in Python.
+    - PostgreSQL: no requirements (see `Time Zones`_).
+    - Oracle: no requirements (see `Choosing a Time Zone File`_).
+    - MySQL: load the time zone tables with `mysql_tzinfo_to_sql`_.
+
+    .. _pytz: http://pytz.sourceforge.net/
+    .. _Time Zones: http://www.postgresql.org/docs/9.2/static/datatype-datetime.html#DATATYPE-TIMEZONES
+    .. _Choosing a Time Zone File: http://docs.oracle.com/cd/B19306_01/server.102/b14225/ch4datetime.htm#i1006667
+    .. _mysql_tzinfo_to_sql: http://dev.mysql.com/doc/refman/5.5/en/mysql-tzinfo-to-sql.html
 
 none
 ~~~~
@@ -2020,7 +2064,7 @@ numbers and even characters.
 year
 ~~~~
 
-For date/datetime fields, exact year match. Takes a four-digit year.
+For date and datetime fields, an exact year match. Takes an integer year.
 
 Example::
 
@@ -2032,6 +2076,9 @@ SQL equivalent::
 
 (The exact SQL syntax varies for each database engine.)
 
+When :setting:`USE_TZ` is ``True``, datetime fields are converted to the
+current time zone before filtering.
+
 .. fieldlookup:: month
 
 month
@@ -2050,12 +2097,15 @@ SQL equivalent::
 
 (The exact SQL syntax varies for each database engine.)
 
+When :setting:`USE_TZ` is ``True``, datetime fields are converted to the
+current time zone before filtering.
+
 .. fieldlookup:: day
 
 day
 ~~~
 
-For date and datetime fields, an exact day match.
+For date and datetime fields, an exact day match. Takes an integer day.
 
 Example::
 
@@ -2070,6 +2120,9 @@ SQL equivalent::
 Note this will match any record with a pub_date on the third day of the month,
 such as January 3, July 3, etc.
 
+When :setting:`USE_TZ` is ``True``, datetime fields are converted to the
+current time zone before filtering.
+
 .. fieldlookup:: week_day
 
 week_day
@@ -2091,12 +2144,74 @@ Note this will match any record with a ``pub_date`` that falls on a Monday (day
 2 of the week), regardless of the month or year in which it occurs. Week days
 are indexed with day 1 being Sunday and day 7 being Saturday.
 
-.. warning::
+When :setting:`USE_TZ` is ``True``, datetime fields are converted to the
+current time zone before filtering.
+
+.. fieldlookup:: hour
+
+hour
+~~~~
+
+.. versionadded:: 1.6
+
+For datetime fields, an exact hour match. Takes an integer between 0 and 23.
+
+Example::
+
+    Event.objects.filter(timestamp__hour=23)
+
+SQL equivalent::
+
+    SELECT ... WHERE EXTRACT('hour' FROM timestamp) = '23';
+
+(The exact SQL syntax varies for each database engine.)
+
+When :setting:`USE_TZ` is ``True``, values are converted to the current time
+zone before filtering.
+
+.. fieldlookup:: minute
+
+minute
+~~~~~~
+
+.. versionadded:: 1.6
+
+For datetime fields, an exact minute match. Takes an integer between 0 and 59.
+
+Example::
+
+    Event.objects.filter(timestamp__minute=29)
+
+SQL equivalent::
+
+    SELECT ... WHERE EXTRACT('minute' FROM timestamp) = '29';
+
+(The exact SQL syntax varies for each database engine.)
+
+When :setting:`USE_TZ` is ``True``, values are converted to the current time
+zone before filtering.
+
+.. fieldlookup:: second
+
+second
+~~~~~~
+
+.. versionadded:: 1.6
+
+For datetime fields, an exact second match. Takes an integer between 0 and 59.
+
+Example::
+
+    Event.objects.filter(timestamp__second=31)
+
+SQL equivalent::
+
+    SELECT ... WHERE EXTRACT('second' FROM timestamp) = '31';
+
+(The exact SQL syntax varies for each database engine.)
 
-    When :doc:`time zone support </topics/i18n/timezones>` is enabled, Django
-    uses UTC in the database connection, which means the ``year``, ``month``,
-    ``day`` and ``week_day`` lookups are performed in UTC. This is a known
-    limitation of the current implementation.
+When :setting:`USE_TZ` is ``True``, values are converted to the current time
+zone before filtering.
 
 .. fieldlookup:: isnull