path: root/Doc/lib/libdatetime.tex

\section{\module{datetime} -- 
         Basic date and time types}

\declaremodule{builtin}{datetime}
\modulesynopsis{Basic date and time types.}
\moduleauthor{Tim Peters}{tim@zope.com} % XXX check address
\sectionauthor{A.M. Kuchling}{amk@amk.ca}

\newcommand{\naive}{na\"ive}

The \module{datetime} module supplies classes for manipulating dates
and times in both simple and complex ways.  While date and time
arithmetic is supported, the focus of the implementation is on
efficient field extraction, for output formatting and manipulation.

There are two kinds of date and time objects: ``\naive'' and ``aware''.
This distinction refers to whether the object has any notion of time
zone, daylight savings time, or other kind of algorithmic or political
time adjustment.  Whether a \naive\ \class{datetime} object represents
Coordinated Universal Time (UTC), local time, or time in some other
timezone is purely up to the program, just like it's up to the program
whether a particular number represents meters, miles, or mass.  \Naive\
\class{datetime} objects are easy to understand and to work with, at
the cost of ignoring some aspects of reality.

For applications requiring more, ``aware'' \class{datetime} subclasses add an
optional time zone information object to the basic \naive\ classes.
These \class{tzinfo} objects capture information about the offset from
UTC time, the time zone name, and whether Daylight Savings Time is in
effect.  Note that no concrete \class{tzinfo} classes are supplied by
the \module{datetime} module.  Instead, they provide a framework for
incorporating the level of detail an app may require.  The rules for
time adjustment across the world are more political than rational, and
there is no standard suitable for every app.

The \module{datetime} module exports the following constants:

\begin{datadesc}{MINYEAR}
    The smallest year number allowed in a \class{date}, 
    \class{datetime}, or  \class{datetimetz} 
    object.  \constant{MINYEAR} is 1.
\end{datadesc}

\begin{datadesc}{MAXYEAR}
    The largest year number allowed in a \class{date}, 
    \class{datetime}, or  \class{datetimetz} 
    object.  \constant{MAXYEAR} is 9999.
\end{datadesc}


\subsection{Available Types}

\begin{classdesc}{date}{}
    An idealized \naive\ date, assuming the current Gregorian calendar
    always was, and always will be, in effect.
    Attributes: \member{year}, \member{month}, and \member{day}.
\end{classdesc}

\begin{classdesc}{time}{}
    An idealized \naive\ time, independent of any particular day, assuming
    that every day has exactly 24*60*60 seconds (there is no notion
    of "leap seconds" here).
    Attributes: \member{hour}, \member{minute}, \member{second}, and 
                \member{microsecond}
\end{classdesc}

\begin{classdesc}{datetime}{}
    A combination of a \naive\ date and a \naive\ time.
    Attributes: \member{year}, \member{month}, \member{day}, 
                \member{hour}, \member{minute}, \member{second}, 
                and \member{microsecond}.
\end{classdesc}

\begin{classdesc}{timedelta}{}
    A duration, expressing the difference between two \class{date},
    \class{time}, or \class{datetime} instances, to microsecond
    resolution.
\end{classdesc}

\begin{classdesc}{tzinfo}{}
    An abstract base class for time zone information objects.  These
    are used by the  \class{datetimetz}  and \class{timetz} classes to
    provided a customizable notion of time adjustment (for example, to
    account for time zone and/or daylight savings time).
\end{classdesc}

\begin{classdesc}{timetz}{}
    An aware subclass of \class{time}, supporting a customizable notion of
    time adjustment.
\end{classdesc}

\begin{classdesc}{datetimetz}{}
    An aware subclass of \class{datetime}, supporting a customizable notion of
    time adjustment.
\end{classdesc}

Objects of these types are immutable.

Objects of the \class{date}, \class{datetime}, and
\class{time} types are always \naive.

An object \code{D} of type \class{timetz} or  \class{datetimetz}  may be
\naive\ or aware.  \code{D} is aware if \code{D.tzinfo} is not \code{None},
and \code{D.tzinfo.utcoffset(D)} does not return \code{None}.  If
\code{D.tzinfo} is \code{None}, or if \code{D.tzinfo} is not
\code{None} but \code{D.tzinfo.utcoffset(D)} returns \code{None}, \code{D} is
\naive.

The distinction between \naive\ and aware doesn't apply to \code{timedelta}
objects.

Subclass relationships
======================
% XXX latex 
    object
        timedelta
        tzinfo
        time
            timetz
        date
            datetime
                datetimetz



\subsection{\method{strftime()} Behavior}

\class{date}, \class{datetime},  \class{datetimetz} , \class{time}, and 
\class{timetz} objects all support
a strftime(format) method, to create a string representing the time
under the control of an explicit format string.  Broadly speaking,
    d.strftime(fmt)
acts like the time module's
    time.strftime(fmt, d.timetuple())
although not all objects support a timetuple() method.

For time and \class{timetz} objects, format codes for year, month, and day
should not be used, as time objects have no such values.  0 is used
instead.

For date objects, format codes for hours, minutes, and seconds should
not be used, as date objects have no such values.  0 is used insted.

For a \naive\ object, the %z and %Z format codes are replaced by
empty strings.

For an aware object:

- %z:  self.utcoffset() is transformed into a 5-character
  string of the form +HHMM or -HHMM, where HH is a 2-digit string
  giving the number of UTC offset hours, and MM is a 2-digit string
  giving the number of UTC offset minutes.  For example, if
  utcoffset() returns -180, %z is replaced with string "-0300".

- %Z:  If self.tzname() returns None, %Z is replaced by an empty string.
  Else %Z is replaced by the returned value, which must be a string.


\subsection{\class{timedelta} \label{datetime-timedelta}

A timedelta object represents a duration, the difference between two
dates or times.

Constructor:

    timedelta(days=0, seconds=0, microseconds=0,
              # The following should only be used as keyword args:
              milliseconds=0, minutes=0, hours=0, weeks=0)

    All arguments are optional.  Arguments may be ints, longs, or floats,
    and may be positive or negative.

    Only days, seconds and microseconds are stored internally.  Arguments
    are converted to those units:

        A millisecond is converted 1000 microseconds.
        A minute is converted to 60 seconds.
        An hour is converted to 3600 seconds.
        A week is converted to 7 days.

    and days, seconds and microseconds are then normalized so that the
    representation is unique, with

        0 <= microseconds < 1000000
        0 <= seconds < 3600*24 (the number of seconds in one day)
        -999999999 <= days <= 999999999

    If any argument is a float, and there are fractional microseconds,
    the fractional microseconds left over from all arguments are combined
    and their sum is rounded to the nearest microsecond.  If no
    argument is a flost, the conversion and normalization processes
    are exact (no information is lost).

    If the normalized value of days lies outside the indicated range,
    OverflowError is raised.

    Note that normalization of negative values may be surprising at first.
    For example,

    >>> d = timedelta(microseconds=-1)
    >>> (d.days, d.seconds, d.microseconds)
    (-1, 86399, 999999)
    >>>


Class attributes:

    .min
        The most negative timedelta object, timedelta(-999999999).

    .max
        The most positive timedelta object,
        timedelta(days=999999999, hours=23, minutes=59, seconds=59,
                  microseconds=999999)

    .resolution
        The smallest possible difference between non-equal timedelta
        objects, timedelta(microseconds=1).

    Note that, because of normalization, timedelta.max > -timedelta.min.
    -timedelta.max is not representable as a timedelta object.

Instance attributes (read-only):

    .days           between -999999999 and 999999999 inclusive
    .seconds        between 0 and 86399 inclusive
    .microseconds   between 0 and 999999 inclusive

Supported operations:

    - timedelta + timedelta -> timedelta
      This is exact, but may overflow.  After
          t1 = t2 + t3
      t1-t2 == t3 and t1-t3 == t2 are true.

    - timedelta - timedelta -> timedelta
      This is exact, but may overflow.  After
          t1 = t2 - t3
       t2 == t1 + t3 is true.

    - timedelta * (int or long) -> timedelta
      (int or long) * timedelta -> timedelta
      This is exact, but may overflow.  After
          t1 = t2 * i
      t1 // i == t2 is true, provided i != 0.  In general,
          t * i == t * (i-1) + t
      is true.

    - timedelta // (int or long) -> timedelta
      The floor is computed and the remainder (if any) is thrown away.
      Division by 0 raises ZeroDivisionError.

    - certain additions and subtractions with date, datetime, and datimetz
      objects (see below)

    - +timedelta -> timedelta
      Returns a timedelta object with the same value.

    - -timedelta -> timedelta
      -t is equivalent to timedelta(-t.days, -t.seconds, -t.microseconds),
      and to t*-1.  This is exact, but may overflow (for example,
      -timedelta.max is not representable as a timedelta object).

    - abs(timedelta) -> timedelta
      abs(t) is equivalent to +t when t.days >= 0, and to -t when
      t.days < 0.  This is exact, and cannot overflow.

    - comparison of timedelta to timedelta; the timedelta representing
      the smaller duration is considered to be the smaller timedelta

    - hash, use as dict key

    - efficient pickling

    - in Boolean contexts, a timedelta object is considred to be true
      if and only if it isn't equal to timedelta(0)


\subsection{\class{date} \label{datetime-date}}

A date object represents a date (year, month and day) in an idealized
calendar, the current Gregorian calendar indefinitely extended in both
directions.  January 1 of year 1 is called day number 1, January 2 of year
1 is called day number 2, and so on.  This matches the definition of the
"proleptic Gregorian" calendar in Dershowitz and Reingold's book
"Calendrical Calculations", where it's the base calendar for all
computations.  See the book for algorithms for converting between
proleptic Gregorian ordinals and many other calendar systems.

Constructor:

    date(year, month, day)

    All arguments are required.  Arguments may be ints or longs, in the
    following ranges:

        MINYEAR <= year <= MAXYEAR
        1 <= month <= 12
        1 <= day <= number of days in the given month and year

    If an argument outside those ranges is given, ValueError is raised.

Other constructors (class methods):

  - today()
    Return the current local date.  This is equivalent to
    date.fromtimestamp(time.time()).

  - fromtimestamp(timestamp)
    Return the local date corresponding to the POSIX timestamp, such as
    is returned by time.time().  This may raise ValueError, if the
    timestamp is out of the range of values supported by the platform C
    localtime() function.  It's common for this to be restricted to
    years in 1970 through 2038.

  - fromordinal(ordinal)
    Return the date corresponding to the proleptic Gregorian ordinal,
    where January 1 of year 1 has ordinal 1.  ValueError is raised
    unless 1 <= ordinal <= date.max.toordinal().  For any date d,
    date.fromordinal(d.toordinal()) == d.

Class attributes:

    .min
        The earliest representable date, date(MINYEAR, 1, 1).

    .max
        The latest representable date, date(MAXYEAR, 12, 31).

    .resolution
        The smallest possible difference between non-equal date
        objects, timedelta(days=1).

Instance attributes (read-only):

    .year           between MINYEAR and MAXYEAR inclusive
    .month          between 1 and 12 inclusive
    .day            between 1 and the number of days in the given month
                    of the given year

Supported operations:

    - date1 + timedelta -> date2
      timedelta + date1 -> date2
      date2 is timedelta.days days removed from the date1, moving forward
      in time if timedelta.days > 0, or backward if timedetla.days < 0.
      date2 - date1 == timedelta.days after.  timedelta.seconds and
      timedelta.microseconds are ignored.  OverflowError is raised if
      date2.year would be smaller than MINYEAR or larger than MAXYEAR.

    - date1 - timedelta -> date2
      Computes the date2 such that date2 + timedelta == date1.  This
      isn't quite equivalent to date1 + (-timedelta), because -timedelta
      in isolation can overflow in cases where date1 - timedelta does
      not.  timedelta.seconds and timedelta.microseconds are ignored.

    - date1 - date2 -> timedelta
      This is exact, and cannot overflow.  timedelta.seconds and
      timedelta.microseconds are 0, and date2 + timedelta == date1
      after.

    - comparison of date to date, where date1 is considered less than
      date2 when date1 precedes date2 in time.  In other words,
      date1 < date2 if and only if date1.toordinal() < date2.toordinal().

    - hash, use as dict key

    - efficient pickling

    - in Boolean contexts, all date objects are considered to be true

Instance methods:

  - timetuple()
    Return a 9-element tuple of the form returned by time.localtime().
    The hours, minutes and seconds are 0, and the DST flag is -1.
    d.timetuple() is equivalent to
        (d.year, d.month, d.day,
         0, 0, 0,  # h, m, s
         d.weekday(),  # 0 is Monday
         d.toordinal() - date(d.year, 1, 1).toordinal() + 1, # day of year
         -1)

  - toordinal()
    Return the proleptic Gregorian ordinal of the date, where January 1
    of year 1 has ordinal 1.  For any date object d,
    date.fromordinal(d.toordinal()) == d.

  - weekday()
    Return the day of the week as an integer, where Monday is 0 and
    Sunday is 6.  For example, date(2002, 12, 4).weekday() == 2, a
    Wednesday.
    See also isoweekday().

  - isoweekday()
    Return the day of the week as an integer, where Monday is 1 and
    Sunday is 7.  For example, date(2002, 12, 4).isoweekday() == 3, a
    Wednesday.
    See also weekday() and isocalendar().

  - isocalendar()
    Return a 3-tuple, (ISO year, ISO week number, ISO weekday).

    The ISO calendar is a widely used variant of the Gregorian calendar.
    See <http://www.phys.uu.nl/~vgent/calendar/isocalendar.htm>
    for a good explanation.

    The ISO year consists of 52 or 53 full weeks, and where a week starts
    on a Monday and ends on a Sunday.  The first week of an ISO year is
    the first (Gregorian) calendar week of a year containing a Thursday.
    This is called week number 1, and the ISO year of that Thursday is
    the same as its Gregorian year.

    For example, 2004 begins on a Thursday, so the first week of ISO
    year 2004 begins on Monday, 29 Dec 2003 and ends on Sunday, 4 Jan
    2004, so that

    date(2003, 12, 29).isocalendar() == (2004, 1, 1)
    date(2004, 1, 4).isocalendar() == (2004, 1, 7)

  - isoformat()
    Return a string representing the date in ISO 8601 format,
    'YYYY-MM-DD'.  For example,
    date(2002, 12, 4).isoformat() == '2002-12-04'.

  - __str__()
    For a date d, str(d) is equivalent to d.isoformat().

  - ctime()
    Return a string representing the date, for example
    date(2002, 12, 4).ctime() == 'Wed Dec  4 00:00:00 2002'.
    d.ctime() is equivalent to time.ctime(time.mktime(d.timetuple()))
    on platforms where the native C ctime() function (which time.ctime()
    invokes, but which date.ctime() does not invoke) conforms to the
    C standard.

  - strftime(format)
    Return a string representing the date, controlled by an explicit
    format string.  Format codes referring to hours, minutes or seconds
    will see 0 values.  See the section on strftime() behavior.


\subsection{\class{datetime} \label{datetime-datetime}}

A \class{datetime} object is a single object containing all the information from
a date object and a time object.  Like a date object, \class{datetime} assumes
the current Gregorian calendar extended in both directions; like a time
object, \class{datetime} assumes there are exactly 3600*24 seconds in every day.

Constructor:

    datetime(year, month, day,
             hour=0, minute=0, second=0, microsecond=0)

    The year, month and day arguments are required.  Arguments may be ints
    or longs, in the following ranges:

        MINYEAR <= year <= MAXYEAR
        1 <= month <= 12
        1 <= day <= number of days in the given month and year
        0 <= hour < 24
        0 <= minute < 60
        0 <= second < 60
        0 <= microsecond < 1000000

    If an argument outside those ranges is given, ValueError is raised.

Other constructors (class methods):

  - today()
    Return the current local datetime.  This is equivalent to
    datetime.fromtimestamp(time.time()).
    See also now(), fromtimestamp().

  - now()
    Return the current local datetime.  This is like today(), but, if
    possible, supplies more precision than can be gotten from going
    through a time.time() timestamp (for example, this may be possible
    on platforms that supply the C gettimeofday() function).
    See also today(), utcnow().

  - utcnow()
    Return the current UTC datetime.  This is like now(), but returns
    the current UTC date and time.
    See also now().

  - fromtimestamp(timestamp)
    Return the local \class{datetime} corresponding to the POSIX timestamp, such
    as is returned by time.time().  This may raise ValueError, if the
    timestamp is out of the range of values supported by the platform C
    localtime() function.  It's common for this to be restricted to
    years in 1970 through 2038.
    See also utcfromtimestamp().

  - utcfromtimestamp(timestamp)
    Return the UTC \class{datetime} corresponding to the POSIX timestamp.
    This may raise ValueError, if the timestamp is out of the range of
    values supported by the platform C gmtime() function.  It's common
    for this to be restricted to years in 1970 through 2038.
    See also fromtimestamp().

  - fromordinal(ordinal)
    Return the \class{datetime} corresponding to the proleptic Gregorian ordinal,
    where January 1 of year 1 has ordinal 1.  ValueError is raised
    unless 1 <= ordinal <= datetime.max.toordinal().  The hour, minute,
    second and microsecond of the result are all 0.

  - combine(date, time)
    Return a new \class{datetime} object whose date components are equal to the
    given date object's, and whose time components are equal to the given
    time object's.  For any \class{datetime} object d,
    d == datetime.combine(d.date(), d.time()).
    If date is a \class{datetime} or \class{datetimetz} object, its time components are
    ignored.  If date is \class{datetimetz} object, its tzinfo component is also
    ignored.  If time is a \class{timetz} object, its tzinfo component is ignored.

Class attributes:

    .min
        The earliest representable datetime,
        datetime(MINYEAR, 1, 1).

    .max
        The latest representable datetime,
        datetime(MAXYEAR, 12, 31, 23, 59, 59, 999999).

    .resolution
        The smallest possible difference between non-equal datetime
        objects, timedelta(microseconds=1).

Instance attributes (read-only):

    .year           between MINYEAR and MAXYEAR inclusive
    .month          between 1 and 12 inclusive
    .day            between 1 and the number of days in the given month
                    of the given year
    .hour           in range(24)
    .minute         in range(60)
    .second         in range(60)
    .microsecond    in range(1000000)

Supported operations:

    - datetime1 + timedelta -> datetime2
      timedelta + datetime1 -> datetime2
      datetime2 is a duration of timedelta removed from datetime1, moving
      forward in time if timedelta.days > 0, or backward if
      timedelta.days < 0.  datetime2 - datetime1 == timedelta after.
      OverflowError is raised if datetime2.year would be smaller than
      MINYEAR or larger than MAXYEAR.

    - datetime1 - timedelta -> datetime2
      Computes the datetime2 such that datetime2 + timedelta == datetime1.
      This isn't quite equivalent to datetime1 + (-timedelta), because
      -timedelta in isolation can overflow in cases where
      datetime1 - timedelta does not.

    - datetime1 - datetime2 -> timedelta
      This is exact, and cannot overflow.
      datetime2 + timedelta == datetime1 after.

    - comparison of \class{datetime} to datetime, where datetime1 is considered
      less than datetime2 when datetime1 precedes datetime2 in time.

    - hash, use as dict key

    - efficient pickling

    - in Boolean contexts, all \class{datetime} objects are considered to be true

Instance methods:

  - date()
    Return date object with same year, month and day.

  - time()
    Return time object with same hour, minute, second and microsecond.

  - timetuple()
    Return a 9-element tuple of the form returned by time.localtime().
    The DST flag is -1.   d.timetuple() is equivalent to
        (d.year, d.month, d.day,
         d.hour, d.minute, d.second,
         d.weekday(),  # 0 is Monday
         d.toordinal() - date(d.year, 1, 1).toordinal() + 1, # day of year
         -1)

  - toordinal()
    Return the proleptic Gregorian ordinal of the date.  The same as
    date.toordinal().

  - weekday()
    Return the day of the week as an integer, where Monday is 0 and
    Sunday is 6.  The same as date.weekday().
    See also isoweekday().

  - isoweekday()
    Return the day of the week as an integer, where Monday is 1 and
    Sunday is 7.  The same as date.isoweekday().
    See also weekday() and isocalendar().

  - isocalendar()
    Return a 3-tuple, (ISO year, ISO week number, ISO weekday).  The
    same as date.isocalendar().

  - isoformat(sep='T')
    Return a string representing the date and time in ISO 8601 format,
        YYYY-MM-DDTHH:MM:SS.mmmmmm
    or, if self.microsecond is 0,
        YYYY-MM-DDTHH:MM:SS
    Optional argument sep (default 'T') is a one-character separator,
    placed between the date and time portions of the result.  For example,
        datetime(2002, 12, 4, 1, 2, 3, 4).isoformat(' ') ==
        '2002-12-04 01:02:03.000004'

  - __str__()
    For a \class{datetime} d, str(d) is equivalent to d.isoformat(' ').

  - ctime()
    Return a string representing the date, for example
    datetime(2002, 12, 4, 20, 30, 40).ctime() == 'Wed Dec  4 20:30:40 2002'.
    d.ctime() is equivalent to time.ctime(time.mktime(d.timetuple()))
    on platforms where the native C ctime() function (which time.ctime()
    invokes, but which datetime.ctime() does not invoke) conforms to the
    C standard.

  - strftime(format)
    Return a string representing the date and time, controlled by an
    explicit format string.  See the section on strftime() behavior.


\subsection{\class{time} \label{datetime-time}}

A time object represents an idealized time of day, independent of day
and timezone.

Constructor:

    time(hour=0, minute=0, second=0, microsecond=0)

    All arguments are optional.  They may be ints or longs, in the
    following ranges:

        0 <= hour < 24
        0 <= minute < 60
        0 <= second < 60
        0 <= microsecond < 1000000

    If an argument outside those ranges is given, ValueError is raised.

Other constructors (class methods):

    None

Class attributes:

    .min
        The earliest representable time, time(0, 0, 0, 0).

    .max
        The latest representable time, time(23, 59, 59, 999999).

    .resolution
        The smallest possible difference between non-equal time
        objects, timedelta(microseconds=1), although note that
        arithmetic on time objects is not supported.

Instance attributes (read-only):

    .hour           in range(24)
    .minute         in range(60)
    .second         in range(60)
    .microsecond    in range(1000000)

Supported operations:

    - comparison of time to time, where time1 is considered
      less than time2 when time1 precedes time2 in time.

    - hash, use as dict key

    - efficient pickling

    - in Boolean contexts, a time object is considered to be true
      if and only if it isn't equal to time(0)

Instance methods:

  - isoformat()
    Return a string representing the time in ISO 8601 format,
        HH:MM:SS.mmmmmm
    or, if self.microsecond is 0
        HH:MM:SS

  - __str__()
    For a time t, str(t) is equivalent to t.isoformat().

  - strftime(format)
    Return a string representing the time, controlled by an explicit
    format string.  See the section on strftime() behavior.


\subsection{\class{tzinfo} \label{datetime-tzinfo}}

tzinfo is an abstract base clase, meaning that objects directly of this
class should not be instantiated.  You need to derive a concrete
subclass, and (at least) supply implementations of the standard tzinfo
methods needed by the \class{datetime} methods you use. The \module{datetime} module does
not supply any concrete subclasses of tzinfo.

An instance of (a concrete subclass of) \class{tzinfo} can be passed to the
constructors for \class{datetimetz} and \class{timetz} objects.  The latter objects
view their fields as being in local time, and the \class{tzinfo} object supports
methods revealing offset of local time from UTC, the name of the time
zone, and DST offset, all relative to a date or time object passed
to them.

A concrete subclass of \class{tzinfo} may need to implement the following
methods.  Exactly which methods are needed depends on the uses made
of aware \class{datetime} objects; if in doubt, simply implement all of them.
The methods are called by a \class{datetimetz} or \class{timetz} object, passing itself
as the argument.  A \class{tzinfo} subclass's methods should be prepared to
accept a dt argument of type None, timetz, or datetimetz.  If is not
None, and dt.tzinfo is not None and not equal to self, an exception
should be raised.

  - utcoffset(dt)
    Return offset of local time from UTC, in minutes east of UTC.  If
    local time is west of UTC, this should be negative.  Note that this
    is intended to be the total offset from UTC; for example, if a
    \class{tzinfo} object represents both time zone and DST adjustments,
    utcoffset() should return their sum.  If the UTC offset isn't known,
    return None.  Else the value returned must be an int (or long), in
    the range -1439 to 1439 inclusive (1440 = 24*60; the magnitude of
    the offset must be less than one day).

  - tzname(dt)
    Return the timezone name corresponding to the \class{datetime} represented
    by dt, as a string.  Nothing about string names is defined by the
    \module{datetime} module, and there's no requirement that it mean anything
    in particular.  For example, "GMT", "UTC", "-500", "-5:00", "EDT",
    "US/Eastern", "America/New York" are all valid replies.  Return
    None if a string name isn't known.  Note that this is a method
    rather than a fixed string primarily because some \class{tzinfo} objects
    will wish to return different names depending on the specific value
    of dt passed, especially if the \class{tzinfo} class is accounting for DST.

  - dst(dt)
    Return the DST offset, in minutes east of UTC, or None if DST
    information isn't known.  Return 0 if DST is not in effect.
    If DST is in effect, return an int (or long), in the range
    -1439 to 1439 inclusive.  Note that DST offset, if applicable,
    has already been added to the UTC offset returned by utcoffset(),
    so there's no need to consult dst() unless you're interested in
    displaying DST info separately.  For example, datetimetz.timetuple()
    calls its \class{tzinfo} object's dst() method to determine how the tm_isdst
    flag should be set.

Example \class{tzinfo} classes:

   class UTC(tzinfo):
        "UTC"
        def utcoffset(self, dt):
            return 0
        def tzname(self, dt):
            return "UTC"
        def dst(self, dt):
            return 0

    class FixedOffset(tzinfo):
        "Fixed offset in minutes east from UTC"
        def __init__(self, offset, name):
            self.__offset = offset
            self.__name = name
        def utcoffset(self, dt):
            return self.__offset
        def tzname(self, dt):
            return self.__name
        def dst(self, dt):
            # It depends on more than we know in an example.
            return None # Indicate we don't know

    import time
    class LocalTime(tzinfo):
        "Local time as defined by the operating system"
        def _isdst(self, dt):
            t = (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second,
                 -1, -1, -1)
            # XXX This may fail for years < 1970 or >= 2038
            t = time.localtime(time.mktime(t))
            return t.tm_isdst > 0
        def utcoffset(self, dt):
            if self._isdst(dt):
                return -time.timezone/60
            else:
                return -time.altzone/60
        def tzname(self, dt):
            return time.tzname[self._isdst(dt)]


\subsection{\class{timetz} \label{datetime-timetz}}

A time object represents a (local) time of day, independent of any
particular day, and subject to adjustment via a \class{tzinfo} object.

Constructor:

    time(hour=0, minute=0, second=0, microsecond=0, tzinfo=None)

    All arguments are optional.  tzinfo may be None, or an instance of
    a \class{tzinfo} subclass.  The remaining arguments may be ints or longs, in
    the following ranges:

        0 <= hour < 24
        0 <= minute < 60
        0 <= second < 60
        0 <= microsecond < 1000000

    If an argument outside those ranges is given, ValueError is raised.

Other constructors (class methods):

    None

Class attributes:

    .min
        The earliest representable time, timetz(0, 0, 0, 0).

    .max
        The latest representable time, timetz(23, 59, 59, 999999).

    .resolution
        The smallest possible difference between non-equal timetz
        objects, timedelta(microseconds=1), although note that
        arithmetic on \class{timetz} objects is not supported.

Instance attributes (read-only):

    .hour           in range(24)
    .minute         in range(60)
    .second         in range(60)
    .microsecond    in range(1000000)
    .tzinfo         the object passed as the tzinfo argument to the
                    \class{timetz} constructor, or None if none was passed.

Supported operations:

    - comparison of \class{timetz} to timetz, where timetz1 is considered
      less than timetz2 when timetz1 precedes timetz2 in time, and
      where the \class{timetz} objects are first adjusted by subtracting
      their UTC offsets (obtained from self.utcoffset()).

    - hash, use as dict key

    - pickling

    - in Boolean contexts, a \class{timetz} object is considered to be true
      if and only if, after converting it to minutes and subtracting
      self.utcoffset() (or 0 if that's None), the result is non-zero.

Instance methods:

  - isoformat()
    Return a string representing the time in ISO 8601 format,
        HH:MM:SS.mmmmmm
    or, if self.microsecond is 0
        HH:MM:SS
    If self.utcoffset() does not return None, a 6-character string is
    appended, giving the UTC offset in (signed) hours and minutes:
        HH:MM:SS.mmmmmm+HH:MM
    or, if self.microsecond is 0
        HH:MM:SS+HH:MM

  - __str__()
    For a \class{timetz} t, str(t) is equivalent to t.isoformat().

  - strftime(format)
    Return a string representing the time, controlled by an explicit
    format string.  See the section on strftime() behavior.

  - utcoffset()
    If self.tzinfo is None, returns None, else self.tzinfo.utcoffset(self).

  - tzname():
    If self.tzinfo is None, returns None, else self.tzinfo.tzname(self).

  - dst()
    If self.tzinfo is None, returns None, else self.tzinfo.dst(self).



\subsection{ \class{datetimetz}  \label{datetime-datetimetz}}

XXX I think this is *still* missing some methods from the
XXX Python implementation.
A \class{datetimetz} object is a single object containing all the information
from a date object and a \class{timetz} object.

Constructor:

    datetimetz(year, month, day,
               hour=0, minute=0, second=0, microsecond=0, tzinfo=None)

    The year, month and day arguments are required.  tzinfo may be None,
    or an instance of a \class{tzinfo} subclass.  The remaining arguments may be
    ints or longs, in the following ranges:

        MINYEAR <= year <= MAXYEAR
        1 <= month <= 12
        1 <= day <= number of days in the given month and year
        0 <= hour < 24
        0 <= minute < 60
        0 <= second < 60
        0 <= microsecond < 1000000

    If an argument outside those ranges is given, ValueError is raised.

Other constructors (class methods):

  - today()
    utcnow()
    utcfromtimestamp(timestamp)
    fromordinal(ordinal)

    These are the same as the \class{datetime} class methods of the same names,
    except that they construct a \class{datetimetz} object, with tzinfo None.

  - now([tzinfo=None])
    fromtimestamp(timestamp[, tzinfo=None])

    These are the same as the \class{datetime} class methods of the same names,
    except that they accept an additional, optional tzinfo argument, and
    construct a \class{datetimetz} object with that \class{tzinfo} object attached.

  - combine(date, time)
    This is the same as datetime.combine(), except that it constructs
    a \class{datetimetz} object, and, if the time object is of type timetz,
    the \class{datetimetz} object has the same \class{tzinfo} object as the time object.

Class attributes:

    .min
        The earliest representable datetimetz,
        datetimetz(MINYEAR, 1, 1).

    .max
        The latest representable datetime,
        datetimetz(MAXYEAR, 12, 31, 23, 59, 59, 999999).

    .resolution
        The smallest possible difference between non-equal datetimetz
        objects, timedelta(microseconds=1).

Instance attributes (read-only):

    .year           between MINYEAR and MAXYEAR inclusive
    .month          between 1 and 12 inclusive
    .day            between 1 and the number of days in the given month
                    of the given year
    .hour           in range(24)
    .minute         in range(60)
    .second         in range(60)
    .microsecond    in range(1000000)
    .tzinfo         the object passed as the tzinfo argument to the
                    \class{datetimetz} constructor, or None if none was passed.

Supported operations:

    - datetimetz1 + timedelta -> datetimetz2
      timedelta + datetimetz1 -> datetimetz2
      The same as addition of \class{datetime} objects, except that
      datetimetz2.tzinfo is set to datetimetz1.tzinfo.

    - datetimetz1 - timedelta -> datetimetz2
      The same as addition of \class{datetime} objects, except that
      datetimetz2.tzinfo is set to datetimetz1.tzinfo.

    - aware_datetimetz1 - aware_datetimetz2 -> timedelta
      \naive\_datetimetz1 - \naive\_datetimetz2 -> timedelta
      \naive\_datetimetz1 - datetime2 -> timedelta
      datetime1 - \naive\_datetimetz2 -> timedelta

      Subtraction of a \class{datetime} or datetimetz, from a \class{datetime} or
      datetimetz, is defined only if both operands are \naive, or if
      both are aware.  If one is aware and the other is \naive, TypeError
      is raised.

      If both are \naive, subtraction acts as for \class{datetime} subtraction.

      If both are aware \class{datetimetz} objects, a-b acts as if a and b were
      first converted to UTC datetimes (by subtracting a.utcoffset()
      minutes from a, and b.utcoffset() minutes from b), and then doing
      \class{datetime} subtraction, except that the implementation never
      overflows.

    - Comparison of \class{datetimetz} to \class{datetime} or datetimetz.  As for
      subtraction, comparison is defined only if both operands are
      \naive\ or both are aware.  If both are \naive, comparison is as
      for \class{datetime} objects with the same date and time components.
      If both are aware, comparison acts as if both were converted to
      UTC datetimes first, except the the implementation never
      overflows.  If one comparand is \naive\ and the other aware,
      TypeError is raised.

    - hash, use as dict key

    - efficient pickling

    - in Boolean contexts, all \class{datetimetz} objects are considered to be
      true

Instance methods:

  - date()
    time()
    toordinal()
    weekday()
    isoweekday()
    isocalendar()
    ctime()
    __str__()
    strftime(format)

    These are the same as the \class{datetime} methods of the same names.

  - timetz()
    Return \class{timetz} object with same hour, minute, second, microsecond,
    and tzinfo.

  - utcoffset()
    If self.tzinfo is None, returns None, else self.tzinfo.utcoffset(self).

  - tzname():
    If self.tzinfo is None, returns None, else self.tzinfo.tzname(self).

  - dst()
    If self.tzinfo is None, returns None, else self.tzinfo.dst(self).

  - timetuple()
    Like datetime.timetuple(), but sets the tm_isdst flag according to
    the dst() method:  if self.dst() returns None, tm_isdst is set to -1;
    else if self.dst() returns a non-zero value, tm_isdst is set to 1;
    else tm_isdst is set to 0.

  - utctimetuple()
    If \class{datetimetz} d is \naive, this is the same as d.timetuple() except
    that tm_isdst is forced to 0 regardless of what d.dst() returns.
    DST is never in effect for a UTC time.

    If d is aware, d is normalized to UTC time, by subtracting
    d.utcoffset() minutes, and a timetuple for the normalized time is
    returned.  tm_isdst is forced to 0.  Note that the result's
    tm_year field may be MINYEAR-1 or MAXYEAR+1, if d.year was MINYEAR
    or MAXYEAR and UTC adjustment spills over a year boundary.

  - isoformat(sep='T')
    Return a string representing the date and time in ISO 8601 format,
        YYYY-MM-DDTHH:MM:SS.mmmmmm
    or, if self.microsecond is 0,
        YYYY-MM-DDTHH:MM:SS

    If self.utcoffset() does not return None, a 6-character string is
    appended, giving the UTC offset in (signed) hours and minutes:
        YYYY-MM-DDTHH:MM:SS.mmmmmm+HH:MM
    or, if self.microsecond is 0
        YYYY-MM-DDTHH:MM:SS+HH:MM

    Optional argument sep (default 'T') is a one-character separator,
    placed between the date and time portions of the result.  For example,

        >>> from \class{datetime} import *
        >>> class TZ(tzinfo):
        ...     def utcoffset(self, dt): return -399
        ...
        >>> datetimetz(2002, 12, 25, tzinfo=TZ()).isoformat(' ')
        '2002-12-25 00:00:00-06:39'
        >>>

    str(d) is equivalent to d.isoformat(' ').


\subsection{C API}

Struct typedefs:

    PyDateTime_Date
    PyDateTime_DateTime
    PyDateTime_DateTimeTZ
    PyDateTime_Time
    PyDateTime_TimeTZ
    PyDateTime_Delta
    PyDateTime_TZInfo

Type-check macros:

    PyDate_Check(op)
    PyDate_CheckExact(op)

    PyDateTime_Check(op)
    PyDateTime_CheckExact(op)

    PyDateTimeTZ_Check(op)
    PyDateTimeTZ_CheckExact(op)

    PyTime_Check(op)
    PyTime_CheckExact(op)

    PyTimeTZ_Check(op)
    PyTimeTZ_CheckExact(op)

    PyDelta_Check(op)
    PyDelta_CheckExact(op)

    PyTZInfo_Check(op)
    PyTZInfo_CheckExact(op

Accessor macros:

All objects are immutable, so accessors are read-only.  All macros
return ints:

    For date, datetime, and \class{datetimetz} instances:
        PyDateTime_GET_YEAR(o)
        PyDateTime_GET_MONTH(o)
        PyDateTime_GET_DAY(o)

    For \class{datetime} and \class{datetimetz} instances:
        PyDateTime_DATE_GET_HOUR(o)
        PyDateTime_DATE_GET_MINUTE(o)
        PyDateTime_DATE_GET_SECOND(o)
        PyDateTime_DATE_GET_MICROSECOND(o)

    For time and \class{timetz} instances:
        PyDateTime_TIME_GET_HOUR(o)
        PyDateTime_TIME_GET_MINUTE(o)
        PyDateTime_TIME_GET_SECOND(o)
        PyDateTime_TIME_GET_MICROSECOND(o)