netcdftime.py

# -*- coding: utf-8 -*-
"""
Performs conversions of netCDF time coordinate data to/from datetime objects.
"""
import math, numpy, re, time
from datetime import datetime as real_datetime

_units = ['days','hours','minutes','seconds','day','hour','minute','second']
_calendars = ['standard','gregorian','proleptic_gregorian','noleap','julian','all_leap','365_day','366_day','360_day']

__version__ = '0.7.2'

class datetime:
    """
Phony datetime object which mimics the python datetime object,
but allows for dates that don't exist in the proleptic gregorian calendar.
Doesn't do timedelta operations, doesn't overload + and -.

Has strftime, timetuple and __repr__ methods.  The format
of the string produced by __repr__ is controlled by self.format
(default %Y-%m-%d %H:%M:%S).

Instance variables are year,month,day,hour,minute,second,dayofwk,dayofyr
and format.
    """
    def __init__(self,year,month,day,hour=0,minute=0,second=0,dayofwk=-1,dayofyr=1):
        """dayofyr set to 1 by default - otherwise time.strftime will complain"""
        self.year=year
        self.month=month
        self.day=day
        self.hour=hour
        self.minute=minute
        self.dayofwk=dayofwk
        self.dayofyr=dayofyr
        self.second=second
        self.format='%Y-%m-%d %H:%M:%S'
    def strftime(self,format=None):
        if format is None:
            format = self.format
        return _strftime(self,format)
    def timetuple(self):
        return (self.year,self.month,self.day,self.hour,self.minute,self.second,self.dayofwk,self.dayofyr,-1)
    def __repr__(self):
        return self.strftime(self.format)

def JulianDayFromDate(date,calendar='standard'):

    """

creates a Julian Day from a 'datetime-like' object.  Returns the fractional
Julian Day (resolution 1 second).

if calendar='standard' or 'gregorian' (default), Julian day follows Julian 
Calendar on and before 1582-10-5, Gregorian calendar after 1582-10-15.

if calendar='proleptic_gregorian', Julian Day follows gregorian calendar.

if calendar='julian', Julian Day follows julian calendar.

Algorithm:

Meeus, Jean (1998) Astronomical Algorithms (2nd Edition). Willmann-Bell,
Virginia. p. 63

    """
    
    # based on redate.py by David Finlayson.

    year=date.year; month=date.month; day=date.day
    hour=date.hour; minute=date.minute; second=date.second
    # Convert time to fractions of a day
    day = day + hour/24.0 + minute/1440.0 + second/86400.0

    # Start Meeus algorithm (variables are in his notation)
    if (month < 3):
        month = month + 12
        year = year - 1
        
    A = int(year/100)

    jd = int(365.25 * (year + 4716)) + int(30.6001 * (month + 1)) + \
         day - 1524.5

    # optionally adjust the jd for the switch from 
    # the Julian to Gregorian Calendar
    # here assumed to have occurred the day after 1582 October 4
    if calendar in ['standard','gregorian']:
        if jd >= 2299170.5:
            # 1582 October 15 (Gregorian Calendar)
            B = 2 - A + int(A/4)
        elif jd < 2299160.5:
            # 1582 October 5 (Julian Calendar)
            B = 0
        else:
            raise ValueError, 'impossible date (falls in gap between end of Julian calendar and beginning of Gregorian calendar'
    elif calendar == 'proleptic_gregorian':
        B = 2 - A + int(A/4)
    elif calendar == 'julian':
        B = 0
    else:
        raise ValueError, 'unknown calendar, must be one of julian,standard,gregorian,proleptic_gregorian, got %s' % calendar
    
    # adjust for Julian calendar if necessary
    jd = jd + B
    
    return jd 

def _NoLeapDayFromDate(date):

    """

creates a Julian Day for a calendar with no leap years from a datetime 
instance.  Returns the fractional Julian Day (resolution 1 second).

    """
    
    year=date.year; month=date.month; day=date.day
    hour=date.hour; minute=date.minute; second=date.second
    # Convert time to fractions of a day
    day = day + hour/24.0 + minute/1440.0 + second/86400.0

    # Start Meeus algorithm (variables are in his notation)
    if (month < 3):
        month = month + 12
        year = year - 1
        
    jd = int(365. * (year + 4716)) + int(30.6001 * (month + 1)) + \
         day - 1524.5
    
    return jd 

def _AllLeapFromDate(date):

    """

creates a Julian Day for a calendar where all years have 366 days from
a 'datetime-like' object.
Returns the fractional Julian Day (resolution 1 second).

    """
    
    year=date.year; month=date.month; day=date.day
    hour=date.hour; minute=date.minute; second=date.second
    # Convert time to fractions of a day
    day = day + hour/24.0 + minute/1440.0 + second/86400.0

    # Start Meeus algorithm (variables are in his notation)
    if (month < 3):
        month = month + 12
        year = year - 1
        
    jd = int(366. * (year + 4716)) + int(30.6001 * (month + 1)) + \
         day - 1524.5
    
    return jd 

def _360DayFromDate(date):

    """

creates a Julian Day for a calendar where all months have 30 daysfrom
a 'datetime-like' object.
Returns the fractional Julian Day (resolution 1 second).

    """
    
    year=date.year; month=date.month; day=date.day
    hour=date.hour; minute=date.minute; second=date.second
    # Convert time to fractions of a day
    day = day + hour/24.0 + minute/1440.0 + second/86400.0

    jd = int(360. * (year + 4716)) + int(30. * (month - 1)) + day
    
    return jd 

def DateFromJulianDay(JD,calendar='standard'):
    """

returns a 'datetime-like' object given Julian Day. Julian Day is a 
fractional day with a resolution of 1 second.

if calendar='standard' or 'gregorian' (default), Julian day follows Julian 
Calendar on and before 1582-10-5, Gregorian calendar after  1582-10-15.

if calendar='proleptic_gregorian', Julian Day follows gregorian calendar.

if calendar='julian', Julian Day follows julian calendar.

The datetime object is a 'real' datetime object if the date falls in
the Gregorian calendar (i.e. calendar='proleptic_gregorian', or
calendar = 'standard'/'gregorian' and the date is after 1582-10-15).
Otherwise, it's a 'phony' datetime object which is actually an instance
of netcdftime.datetime.


Algorithm:

Meeus, Jean (1998) Astronomical Algorithms (2nd Edition). Willmann-Bell,
Virginia. p. 63

    """

    # based on redate.py by David Finlayson.
    
    if JD < 0:
        raise ValueError, 'Julian Day must be positive'

    dayofwk = int(math.fmod(int(JD + 1.5),7))
    (F, Z) = math.modf(JD + 0.5)
    Z = int(Z)
    if calendar in ['standard','gregorian']:
        if JD < 2299160.5:
            A = Z
        else:
            alpha = int((Z - 1867216.25)/36524.25)
            A = Z + 1 + alpha - int(alpha/4)

    elif calendar == 'proleptic_gregorian':
        alpha = int((Z - 1867216.25)/36524.25)
        A = Z + 1 + alpha - int(alpha/4)
    elif calendar == 'julian':
        A = Z
    else:
        raise ValueError, 'unknown calendar, must be one of julian,standard,gregorian,proleptic_gregorian, got %s' % calendar

    B = A + 1524
    C = int((B - 122.1)/365.25)
    D = int(365.25 * C)
    E = int((B - D)/30.6001)

    # Convert to date
    day = B - D - int(30.6001 * E) + F
    nday = B-D-123
    if nday <= 305:
        dayofyr = nday+60
    else:
        dayofyr = nday-305
    if E < 14:
        month = E - 1
    else:
        month = E - 13

    if month > 2:
        year = C - 4716
    else:
        year = C - 4715

    # a leap year?
    leap = 0
    if year % 4 == 0:
        leap = 1
    if calendar == 'proleptic_gregorian' or \
       (calendar in ['standard','gregorian'] and JD >= 2299160.5):
        if year % 100 == 0 and year % 400 != 0: 
            print year % 100, year % 400
            leap = 0
    if leap and month > 2:
       dayofyr = dayofyr + leap
    
    # Convert fractions of a day to time    
    (dfrac, days) = math.modf(day/1.0)
    (hfrac, hours) = math.modf(dfrac * 24.0)
    (mfrac, minutes) = math.modf(hfrac * 60.0)
    seconds = round(mfrac * 60.0) # seconds are rounded
    
    if seconds > 59:
        seconds = 0
        minutes = minutes + 1
    if minutes > 59:
        minutes = 0
        hours = hours + 1
    if hours > 23:
        hours = 0
        days = days + 1
    
    # return a 'real' datetime instance if calendar is gregorian.
    if calendar == 'proleptic_gregorian' or \
            (calendar in ['standard','gregorian'] and JD >= 2299160.5):
        return real_datetime(year,month,int(days),int(hours),int(minutes),int(seconds))
    else:
    # or else, return a 'datetime-like' instance.
        return datetime(year,month,int(days),int(hours),int(minutes),int(seconds),dayofwk,dayofyr)

def _DateFromNoLeapDay(JD):
    """

returns a 'datetime-like' object given Julian Day for a calendar with no leap 
days. Julian Day is a fractional day with a resolution of 1 second.

    """

    # based on redate.py by David Finlayson.
    
    if JD < 0:
        raise ValueError, 'Julian Day must be positive'

    dayofwk = int(math.fmod(int(JD + 1.5),7))
    (F, Z) = math.modf(JD + 0.5)
    Z = int(Z)
    A = Z
    B = A + 1524
    C = int((B - 122.1)/365.)
    D = int(365. * C)
    E = int((B - D)/30.6001)

    # Convert to date
    day = B - D - int(30.6001 * E) + F
    nday = B-D-123
    if nday <= 305:
        dayofyr = nday+60
    else:
        dayofyr = nday-305
    if E < 14:
        month = E - 1
    else:
        month = E - 13

    if month > 2:
        year = C - 4716
    else:
        year = C - 4715
    
    # Convert fractions of a day to time    
    (dfrac, days) = math.modf(day/1.0)
    (hfrac, hours) = math.modf(dfrac * 24.0)
    (mfrac, minutes) = math.modf(hfrac * 60.0)
    seconds = round(mfrac * 60.0) # seconds are rounded
    
    if seconds > 59:
        seconds = 0
        minutes = minutes + 1
    if minutes > 59:
        minutes = 0
        hours = hours + 1
    if hours > 23:
        hours = 0
        days = days + 1
    
    return datetime(year,month,int(days),int(hours),int(minutes),int(seconds), dayofwk, dayofyr)

def _DateFromAllLeap(JD):
    """

returns a 'datetime-like' object given Julian Day for a calendar where all
years have 366 days.
Julian Day is a fractional day with a resolution of 1 second.

    """

    # based on redate.py by David Finlayson.
    
    if JD < 0:
        raise ValueError, 'Julian Day must be positive'

    dayofwk = int(math.fmod(int(JD + 1.5),7))
    (F, Z) = math.modf(JD + 0.5)
    Z = int(Z)
    A = Z
    B = A + 1524
    C = int((B - 122.1)/366.)
    D = int(366. * C)
    E = int((B - D)/30.6001)

    # Convert to date
    day = B - D - int(30.6001 * E) + F
    nday = B-D-123
    if nday <= 305:
        dayofyr = nday+60
    else:
        dayofyr = nday-305
    if E < 14:
        month = E - 1
    else:
        month = E - 13
    if month > 2:
       dayofyr = dayofyr+1

    if month > 2:
        year = C - 4716
    else:
        year = C - 4715
    
    # Convert fractions of a day to time    
    (dfrac, days) = math.modf(day/1.0)
    (hfrac, hours) = math.modf(dfrac * 24.0)
    (mfrac, minutes) = math.modf(hfrac * 60.0)
    seconds = round(mfrac * 60.0) # seconds are rounded
    
    if seconds > 59:
        seconds = 0
        minutes = minutes + 1
    if minutes > 59:
        minutes = 0
        hours = hours + 1
    if hours > 23:
        hours = 0
        days = days + 1
    
    return datetime(year,month,int(days),int(hours),int(minutes),int(seconds), dayofwk, dayofyr)

def _DateFrom360Day(JD):
    """

returns a 'datetime-like' object given Julian Day for a calendar where all
months have 30 days.
Julian Day is a fractional day with a resolution of 1 second.

    """

    if JD < 0:
        raise ValueError, 'Julian Day must be positive'

    #jd = int(360. * (year + 4716)) + int(30. * (month - 1)) + day
    (F, Z) = math.modf(JD)
    year = int((Z-0.5)/360.) - 4716
    dayofyr =  JD - (year+4716)*360  
    month = int((dayofyr-0.5)/30)+1
    day = dayofyr - (month-1)*30 + F  
    
    # Convert fractions of a day to time    
    (dfrac, days) = math.modf(day/1.0)
    (hfrac, hours) = math.modf(dfrac * 24.0)
    (mfrac, minutes) = math.modf(hfrac * 60.0)
    seconds = round(mfrac * 60.0) # seconds are rounded
    
    if seconds > 59:
        seconds = 0
        minutes = minutes + 1
    if minutes > 59:
        minutes = 0
        hours = hours + 1
    if hours > 23:
        hours = 0
        days = days + 1
    
    return datetime(year,month,int(days),int(hours),int(minutes),int(seconds),-1, int(dayofyr))

def _dateparse(timestr):
    """parse a string of the form time-units since yyyy-mm-dd hh:mm:ss
    return a tuple (units, datetimeinstance)"""
    timestr_split = timestr.split()
    units = timestr_split[0].lower()
    if units not in _units:
        raise ValueError,"units must be one of 'seconds', 'minutes', 'hours' or 'days' (or singular version of these), got '%s'" % units
    if timestr_split[1].lower() != 'since':
        raise ValueError,"no 'since' in unit_string"
    # parse the date string.
    n = timestr.find('since')+6
    year,month,day,hour,minute,second,utc_offset = _parse_date(timestr[n:])
    return units, utc_offset, datetime(year, month, day, hour, minute, second)

class utime:
    """
Performs conversions of netCDF time coordinate
data to/from datetime objects.

To initialize: C{t = utime(unit_string,calendar='standard')}

where 

B{C{unit_string}} is a string of the form
C{'time-units since <time-origin>'} defining the time units.

Valid time-units are days, hours, minutes and seconds (the singular forms 
are also accepted). An example unit_string would be C{'hours 
since 0001-01-01 00:00:00'}.

The B{C{calendar}} keyword describes the calendar used in the time calculations. 
All the values currently defined in the U{CF metadata convention 
<http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.1/cf-conventions.html#time-coordinate>}  
are accepted. The default is C{'standard'}, which corresponds to the mixed 
Gregorian/Julian calendar used by the C{udunits library}. Valid calendars 
are:

C{'gregorian'} or C{'standard'} (default):

Mixed Gregorian/Julian calendar as defined by udunits.

C{'proleptic_gregorian'}:

A Gregorian calendar extended to dates before 1582-10-15. That is, a year 
is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) 
it is divisible by 400.

C{'noleap'} or C{'365_day'}:

Gregorian calendar without leap years, i.e., all years are 365 days long. 
all_leap or 366_day Gregorian calendar with every year being a leap year, 
i.e., all years are 366 days long.

C{'360_day'}:

All years are 360 days divided into 30 day months. 

C{'julian'}:

Proleptic Julian calendar, extended to dates after 1582-10-5. A year is a 
leap year if it is divisible by 4.

The C{L{num2date}} and C{L{date2num}} class methods can used to convert datetime 
instances to/from the specified time units using the specified calendar.

The datetime instances returned by C{num2date} are 'real' python datetime 
objects if the date falls in the Gregorian calendar (i.e. 
C{calendar='proleptic_gregorian', 'standard'} or C{'gregorian'} and 
the date is after 1582-10-15). Otherwise, they are 'phony' datetime 
objects which are actually instances of C{L{netcdftime.datetime}}.  This is 
because the python datetime module cannot handle the weird dates in some 
calendars (such as C{'360_day'} and C{'all_leap'}) which don't exist in any real 
world calendar.


Example usage:

>>> from netcdftime import utime
>>> from datetime import  datetime
>>> cdftime = utime('hours since 0001-01-01 00:00:00')
>>> date = datetime.now()
>>> print date
2006-03-17 16:04:02.561678
>>>
>>> t = cdftime.date2num(date)
>>> print t
17577328.0672
>>>
>>> date = cdftime.num2date(t)
>>> print date
2006-03-17 16:04:02
>>>

The resolution of the transformation operation is 1 second.
        
Warning:  Dates between 1582-10-5 and 1582-10-15 do not exist in the 
C{'standard'} or C{'gregorian'} calendars.  An exception will be raised if you pass 
a 'datetime-like' object in that range to the C{L{date2num}} class method.

Words of Wisdom from the British MetOffice concerning reference dates:

"udunits implements the mixed Gregorian/Julian calendar system, as 
followed in England, in which dates prior to 1582-10-15 are assumed to use 
the Julian calendar. Other software cannot be relied upon to handle the 
change of calendar in the same way, so for robustness it is recommended 
that the reference date be later than 1582. If earlier dates must be used, 
it should be noted that udunits treats 0 AD as identical to 1 AD."

@ivar origin: datetime instance defining the origin of the netCDF time variable.
@ivar calendar:  the calendar used (as specified by the C{calendar} keyword).
@ivar unit_string:  a string defining the the netCDF time variable.
@ivar units:  the units part of C{unit_string} (i.e. 'days', 'hours', 'seconds').
    """
    def __init__(self,unit_string,calendar='standard'):
        """
@param unit_string: a string of the form
C{'time-units since <time-origin>'} defining the time units.

Valid time-units are days, hours, minutes and seconds (the singular forms 
are also accepted). An example unit_string would be C{'hours 
since 0001-01-01 00:00:00'}.

@keyword calendar: describes the calendar used in the time calculations. 
All the values currently defined in the U{CF metadata convention 
<http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.1/cf-conventions.html#time-coordinate>}
are accepted. The default is C{'standard'}, which corresponds to the mixed 
Gregorian/Julian calendar used by the C{udunits library}. Valid calendars 
are:
 - C{'gregorian'} or C{'standard'} (default):
 Mixed Gregorian/Julian calendar as defined by udunits.
 - C{'proleptic_gregorian'}:
 A Gregorian calendar extended to dates before 1582-10-15. That is, a year 
 is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) 
 it is divisible by 400.
 - C{'noleap'} or C{'365_day'}:
 Gregorian calendar without leap years, i.e., all years are 365 days long. 
 - C{'all_leap'} or C{'366_day'}:  
 Gregorian calendar with every year being a leap year, i.e.,
 all years are 366 days long.
 -C{'360_day'}:
 All years are 360 days divided into 30 day months. 
 -C{'julian'}:
 Proleptic Julian calendar, extended to dates after 1582-10-5. A year is a 
 leap year if it is divisible by 4.

@returns: A class instance which may be used for converting times from netCDF
units to datetime objects.
        """
        if calendar in _calendars:
            self.calendar = calendar
        else:
            raise ValueError, "calendar must be one of %s, got '%s'" % (str(_calendars),calendar)
        units, tzoffset, self.origin = _dateparse(unit_string)
        self.tzoffset = tzoffset # time zone offset in minutes
        self.units = units
        self.unit_string = unit_string
        if self.calendar in ['noleap','365_day'] and self.origin.month == 2 and self.origin.day == 29:
            raise ValueError, 'cannot specify a leap day as the reference time with the noleap calendar'
        if self.calendar == '360_day' and self.origin.day > 30:
            raise ValueError, 'there are only 30 days in every month with the 360_day calendar'
        if self.calendar in ['noleap','365_day']:
            self._jd0 = _NoLeapDayFromDate(self.origin)
        elif self.calendar in ['all_leap','366_day']:
            self._jd0 = _AllLeapFromDate(self.origin)
        elif self.calendar == '360_day':
            self._jd0 = _360DayFromDate(self.origin)
        else:
            self._jd0 = JulianDayFromDate(self.origin,calendar=self.calendar)

    def date2num(self,date):
        """
Returns C{time_value} in units described by L{unit_string}, using
the specified L{calendar}, given a 'datetime-like' object.

The datetime object must represent UTC with no time-zone offset.
If there is a time-zone offset implied by L{unit_string}, it will
be applied to the returned numeric values.

Resolution is 1 second.

If C{calendar = 'standard'} or C{'gregorian'} (indicating
that the mixed Julian/Gregorian calendar is to be used), an
exception will be raised if the 'datetime-like' object describes
a date between 1582-10-5 and 1582-10-15.

Works for scalars, sequences and numpy arrays.
Returns a scalar if input is a scalar, else returns a numpy array.
        """
        isscalar = False
        try:
            date[0]
        except:
            isscalar = True
        if not isscalar:
            date = numpy.array(date)
            shape = date.shape
        if self.calendar in ['julian','standard','gregorian','proleptic_gregorian']:
            if isscalar:
                jdelta = JulianDayFromDate(date,self.calendar)-self._jd0
            else:
                jdelta = [JulianDayFromDate(d,self.calendar)-self._jd0 for d in date.flat]
        elif self.calendar in ['noleap','365_day']:
            if date.month == 2 and date.day == 29:
                raise ValueError, 'there is no leap day in the noleap calendar'
            if isscalar:
                jdelta = _NoLeapDayFromDate(date) - self._jd0
            else:
                jdelta = [_NoLeapDayFromDate(d)-self._jd0 for d in date.flat]
        elif self.calendar in ['all_leap','366_day']:
            if isscalar:
                jdelta = _AllLeapFromDate(date) - self._jd0
            else:
                jdelta = [_AllLeapFromDate(d)-self._jd0 for d in date.flat]
        elif self.calendar == '360_day':
            if self.calendar == '360_day' and date.day > 30:
                raise ValueError, 'there are only 30 days in every month with the 360_day calendar'
            if isscalar:
                jdelta = _360DayFromDate(date) - self._jd0
            else:
                jdelta = [_360DayFromDate(d)-self._jd0 for d in date.flat]
        if not isscalar:
            jdelta = numpy.array(jdelta)
        # convert to desired units, add time zone offset.
        if self.units in ['second','seconds']:
            jdelta = jdelta*86400. + self.tzoffset*60.
        elif self.units in ['minute','minutes']:
            jdelta = jdelta*1440. + self.tzoffset
        elif self.units in ['hour','hours']:
            jdelta = jdelta*24. + self.tzoffset/60.
        elif self.units in ['day','days']:
            jdelta = jdelta + self.tzoffset/1440.
        if isscalar:
            return jdelta
        else:
            return numpy.reshape(jdelta,shape)

    def num2date(self,time_value):
        """
Return a 'datetime-like' object given a C{time_value} in units
described by L{unit_string}, using L{calendar}.

dates are in UTC with no offset, even if L{unit_string} contains
a time zone offset from UTC.

Resolution is 1 second.

Works for scalars, sequences and numpy arrays.
Returns a scalar if input is a scalar, else returns a numpy array.

The datetime instances returned by C{num2date} are 'real' python datetime 
objects if the date falls in the Gregorian calendar (i.e. 
C{calendar='proleptic_gregorian'}, or C{calendar = 'standard'/'gregorian'} and 
the date is after 1582-10-15). Otherwise, they are 'phony' datetime 
objects which are actually instances of netcdftime.datetime.  This is 
because the python datetime module cannot handle the weird dates in some 
calendars (such as C{'360_day'} and C{'all_leap'}) which 
do not exist in any real world calendar.
        """
        isscalar = False
        try:
            time_value[0]
        except:
            isscalar = True
        if not isscalar:
            time_value = numpy.array(time_value, dtype='d')
            shape = time_value.shape
        # convert to desired units, remove time zone offset.
        if self.units in ['second','seconds']:
            jdelta = time_value/86400. - self.tzoffset/1440.
        elif self.units in ['minute','minutes']:
            jdelta = time_value/1440. - self.tzoffset/1440.
        elif self.units in ['hour','hours']:
            jdelta = time_value/24. - self.tzoffset/1440.
        elif self.units in ['day','days']:
            jdelta = time_value - self.tzoffset/1440.
        jd = self._jd0 + jdelta
        if self.calendar in ['julian','standard','gregorian','proleptic_gregorian']:
            if not isscalar:
                date = [DateFromJulianDay(j,self.calendar) for j in jd.flat]
            else:
                date = DateFromJulianDay(jd,self.calendar)
        elif self.calendar in ['noleap','365_day']:
            if not isscalar:
                date = [_DateFromNoLeapDay(j) for j in jd.flat]
            else:
                date = _DateFromNoLeapDay(jd)
        elif self.calendar in ['all_leap','366_day']:
            if not isscalar:
                date = [_DateFromAllLeap(j) for j in jd.flat]
            else:
                date = _DateFromAllLeap(jd)
        elif self.calendar == '360_day':
            if not isscalar:
                date = [_DateFrom360Day(j) for j in jd.flat]
            else:
                date = _DateFrom360Day(jd)
        if isscalar:
            return date
        else:
            return numpy.reshape(numpy.array(date),shape)

def _parse_date(origin):
    """Parses a date string and returns a tuple
    (year,month,day,hour,minute,second,utc_offset).
    utc_offset is in minutes.

    This function parses the 'origin' part of the time unit. It should be
    something like::

        2004-11-03 14:42:27.0 +2:00

    Lots of things are optional; just the date is mandatory.

    by Roberto De Almeida

    excerpted from coards.py - http://cheeseshop.python.org/pypi/coards/
    """
    # yyyy-mm-dd [hh:mm:ss[.s][ [+-]hh[:][mm]]]
    p = re.compile( r'''(?P<year>\d{1,4})           # yyyy
                        -                           #
                        (?P<month>\d{1,2})          # mm or m
                        -                           #
                        (?P<day>\d{1,2})            # dd or d
                                                    #
                        (?:                         # [optional time and timezone]
                            \s                      #
                            (?P<hour>\d{1,2})       #   hh or h
                            :                       #
                            (?P<min>\d{1,2})        #   mm or m
                            (?:
                            \:
                            (?P<sec>\d{1,2})        #   ss or s (optional)
                            )?
                                                    #
                            (?:                     #   [optional decisecond]
                                \.                  #       .
                                (?P<dsec>\d)        #       s
                            )?                      #
                            (?:                     #   [optional timezone]
                                \s                  #
                                (?P<ho>[+-]?        #       [+ or -]
                                \d{1,2})            #       hh or h
                                :?                  #       [:]
                                (?P<mo>\d{2})?      #       [mm]
                            )?                      #
                        )?                          #
                        $                           # EOL
                    ''', re.VERBOSE)

    m = p.match(origin.strip())
    if m:
        c = m.groupdict(0)
        # UTC offset.
        offset = int(c['ho'])*60 + int(c['mo'])
        return int(c['year']),int(c['month']),int(c['day']),int(c['hour']),int(c['min']),int(c['sec']),offset
    
    raise Exception('Invalid date origin: %s' % origin)

# remove the unsupposed "%s" command.  But don't
# do it if there's an even number of %s before the s
# because those are all escaped.  Can't simply
# remove the s because the result of
#  %sY
# should be %Y if %s isn't supported, not the
# 4 digit year.
_illegal_s = re.compile(r"((^|[^%])(%%)*%s)")

def _findall(text, substr):
     # Also finds overlaps
     sites = []
     i = 0
     while 1:
         j = text.find(substr, i)
         if j == -1:
             break
         sites.append(j)
         i=j+1
     return sites

# Every 28 years the calendar repeats, except through century leap
# years where it's 6 years.  But only if you're using the Gregorian
# calendar.  ;)

def _strftime(dt, fmt):
    if _illegal_s.search(fmt):
        raise TypeError("This strftime implementation does not handle %s")
    # don't use strftime method at all.
    #if dt.year > 1900:
    #    return dt.strftime(fmt)

    year = dt.year
    # For every non-leap year century, advance by
    # 6 years to get into the 28-year repeat cycle
    delta = 2000 - year
    off = 6*(delta // 100 + delta // 400)
    year = year + off

    # Move to around the year 2000
    year = year + ((2000 - year)//28)*28
    timetuple = dt.timetuple()
    s1 = time.strftime(fmt, (year,) + timetuple[1:])
    sites1 = _findall(s1, str(year))
    
    s2 = time.strftime(fmt, (year+28,) + timetuple[1:])
    sites2 = _findall(s2, str(year+28))

    sites = []
    for site in sites1:
        if site in sites2:
            sites.append(site)
            
    s = s1
    syear = "%4d" % (dt.year,)
    for site in sites:
        s = s[:site] + syear + s[site+4:]
    return s

def date2num(dates,units,calendar='standard'):
    """
date2num(dates,units,calendar='standard')

Return numeric time values given datetime objects. The units
of the numeric time values are described by the L{units} argument
and the L{calendar} keyword. The datetime objects must
be in UTC with no time-zone offset.  If there is a 
time-zone offset in C{units}, it will be applied to the
returned numeric values.

Like the matplotlib C{date2num} function, except that it allows
for different units and calendars.  Behaves the same if
C{units = 'days since 0001-01-01 00:00:00'} and 
C{calendar = 'proleptic_gregorian'}.

@param dates: A datetime object or a sequence of datetime objects.
 The datetime objects should not include a time-zone offset.

@param units: a string of the form C{'B{time units} since B{reference time}}'
 describing the time units. B{C{time units}} can be days, hours, minutes
 or seconds.  B{C{reference time}} is the time origin. A valid choice
 would be units=C{'hours since 1800-01-01 00:00:00 -6:00'}.

@param calendar: describes the calendar used in the time calculations. 
 All the values currently defined in the U{CF metadata convention 
 <http://cf-pcmdi.llnl.gov/documents/cf-conventions/>} are supported.
 Valid calendars C{'standard', 'gregorian', 'proleptic_gregorian'
 'noleap', '365_day', '360_day', 'julian', 'all_leap', '366_day'}.
 Default is C{'standard'}, which is a mixed Julian/Gregorian calendar.

@return: a numeric time value, or an array of numeric time values.

The maximum resolution of the numeric time values is 1 second.
    """
    cdftime = utime(units,calendar=calendar)
    return cdftime.date2num(dates)

def num2date(times,units,calendar='standard'):
    """
num2date(times,units,calendar='standard')

Return datetime objects given numeric time values. The units
of the numeric time values are described by the C{units} argument
and the C{calendar} keyword. The returned datetime objects represent 
UTC with no time-zone offset, even if the specified 
C{units} contain a time-zone offset.

Like the matplotlib C{num2date} function, except that it allows
for different units and calendars.  Behaves the same if
C{units = 'days since 001-01-01 00:00:00'} and 
C{calendar = 'proleptic_gregorian'}.

@param times: numeric time values. Maximum resolution is 1 second.

@param units: a string of the form C{'B{time units} since B{reference time}}'
describing the time units. B{C{time units}} can be days, hours, minutes
or seconds.  B{C{reference time}} is the time origin. A valid choice
would be units=C{'hours since 1800-01-01 00:00:00 -6:00'}.

@param calendar: describes the calendar used in the time calculations. 
All the values currently defined in the U{CF metadata convention 
<http://cf-pcmdi.llnl.gov/documents/cf-conventions/>} are supported.
Valid calendars C{'standard', 'gregorian', 'proleptic_gregorian'
'noleap', '365_day', '360_day', 'julian', 'all_leap', '366_day'}.
Default is C{'standard'}, which is a mixed Julian/Gregorian calendar.

@return: a datetime instance, or an array of datetime instances.

The datetime instances returned are 'real' python datetime 
objects if the date falls in the Gregorian calendar (i.e. 
C{calendar='proleptic_gregorian'}, or C{calendar = 'standard'} or C{'gregorian'}
and the date is after 1582-10-15). Otherwise, they are 'phony' datetime 
objects which support some but not all the methods of 'real' python
datetime objects.  This is because the python datetime module cannot
the uses the C{'proleptic_gregorian'} calendar, even before the switch
occured from the Julian calendar in 1582. The datetime instances
do not contain a time-zone offset, even if the specified C{units}
contains one.
    """
    cdftime = utime(units,calendar=calendar)
    return cdftime.num2date(times)

def _check_index(indices, dates, nctime, calendar):
    """Return True if the time indices given correspond to the given dates, 
    False otherwise.
    
    Parameters: 

    indices : sequence of integers
    Positive integers indexing the time variable.

    dates : sequence of datetime objects
    Reference dates.

    nctime : netCDF Variable object
    NetCDF time object.

    calendar : string
    Calendar of nctime.
     """
    if  (indices <0).any():
       return False
       
    if (indices >= nctime.shape[0]).any():
        return False
        
    t = nctime[indices] 
# if fancy indexing not available, fall back on this.
#   t=[]
#   for ind in indices:
#       t.append(nctime[ind])
    return numpy.all( num2date(t, nctime.units, calendar) == dates)



def date2index(dates, nctime, calendar=None, select='exact'):
    """
    date2index(dates, nctime, calendar=None, select='exact')
    
    Return indices of a netCDF time variable corresponding to the given dates.
    
    @param dates: A datetime object or a sequence of datetime objects.
    The datetime objects should not include a time-zone offset.
    
    @param nctime: A netCDF time variable object. The nctime object must have a
    C{units} attribute. The entries are assumed to be stored in increasing 
    order.
    
    @param calendar: Describes the calendar used in the time calculation.
    Valid calendars C{'standard', 'gregorian', 'proleptic_gregorian'
    'noleap', '365_day', '360_day', 'julian', 'all_leap', '366_day'}.
    Default is C{'standard'}, which is a mixed Julian/Gregorian calendar
    If C{calendar} is None, its value is given by C{nctime.calendar} or
    C{standard} if no such attribute exists.
    
    @param select: C{'exact', 'before', 'after', 'nearest'}
    The index selection method. C{exact} will return the indices perfectly 
    matching the dates given. C{before} and C{after} will return the indices 
    corresponding to the dates just before or just after the given dates if 
    an exact match cannot be found. C{nearest} will return the indices that 
    correpond to the closest dates. 
    """
    # Setting the calendar.
    if calendar == None:
        calendar = getattr(nctime, 'calendar', 'standard')
   
    num = numpy.atleast_1d(date2num(dates, nctime.units, calendar))
    N = len(nctime)
    
    # Trying to infer the correct index from the starting time and the stride.
    # This assumes that the times are increasing uniformly.
    t0, t1 = nctime[:2]
    dt = t1 - t0
    index = numpy.array((num-t0)/dt, int)

    # Checking that the index really corresponds to the given date.
    # If the times do not correspond, then it means that the times
    # are not increasing uniformly and we try the bisection method.
    if not _check_index(index, dates, nctime, calendar):
        
        # Use the bisection method. Assumes the dates are ordered.
        import bisect
        index = numpy.array([bisect.bisect_left(nctime, n) for n in num], int)
               
        after = index == N
        before = index == 0
        
        if select in ['before', 'exact'] and numpy.any(before):
            raise ValueError, 'At least one of the dates given is before the first date in `nctime`.'
        
        if select in ['after', 'exact'] and numpy.any(after):
            raise ValueError, 'At least one of the dates given is after the last date in `nctime`.'
            
        
        # Find the dates for which the match is not perfect.
        # Use list comprehension instead of the simpler `nctime[index]` since
        # not all time objects support numpy integer indexing (eg dap).
        index[after] = N-1
        ncnum = numpy.squeeze([nctime[i] for i in index])
        mismatch = numpy.nonzero(ncnum != num)[0]
                   
        if select == 'exact':
            if len(mismatch) > 0:
                raise ValueError, 'Some of the dates specified were not found in the `nctime` variable.'
   
        elif select == 'before':
            index[after] = N
            index[mismatch] -= 1
               
        elif select == 'after':
            pass
       
        elif select == 'nearest':
            nearest_to_left = num[mismatch] < numpy.array( [nctime[i-1] + nctime[i] for i in index[mismatch]]) / 2.
            index[mismatch] = index[mismatch] - 1 * nearest_to_left
       
        else:
            raise ValueError("%s is not an option for the `select` argument."%select)
   

        # Correct for indices equal to -1
        index[before] = 0
        
    # convert numpy scalars or single element arrays to python ints.
    return _toscalar(index)


def _toscalar(a):
    if a.shape in [(),(1,)]:
        return a.item()
    else:
        return a