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Performs conversions of netCDF time coordinate data to/from datetime objects.
To initialize: t =
utime(unit_string,calendar='standard')
where
unit_string
is a string of the form
'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 'hours
since 0001-01-01 00:00:00'
.
The calendar
keyword describes the calendar used
in the time calculations. All the values currently defined in the CF metadata convention are accepted. The default is
'standard'
, which corresponds to the mixed Gregorian/Julian
calendar used by the udunits library
. Valid calendars
are:
'gregorian'
or 'standard'
(default):
Mixed Gregorian/Julian calendar as defined by udunits.
'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.
'noleap'
or '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.
'360_day'
:
All years are 360 days divided into 30 day months.
'julian'
:
Proleptic Julian calendar, extended to dates after 1582-10-5. A year is a leap year if it is divisible by 4.
The num2date
and date2num
class methods can used to convert
datetime instances to/from the specified time units using the specified
calendar.
The datetime instances returned by num2date
are 'real'
python datetime objects if the date falls in the Gregorian calendar (i.e.
calendar='proleptic_gregorian', 'standard'
or
'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
'360_day'
and '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
'standard'
or 'gregorian'
calendars. An
exception will be raised if you pass a 'datetime-like' object in that
range to the 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."
Instance Methods | |||
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Instance Variables | |
calendar the calendar used (as specified by the calendar
keyword).
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origin datetime instance defining the origin of the netCDF time variable. |
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unit_string a string defining the the netCDF time variable. |
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units the units part of unit_string (i.e.
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Method Details |
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Returns The datetime object must represent UTC with no time-zone offset. If there is a time-zone offset implied by unit_string, it will be applied to the returned numeric values. Resolution is 1 second. If Works for scalars, sequences and numpy arrays. Returns a scalar if input is a scalar, else returns a numpy array. |
Return a 'datetime-like' object given a dates are in UTC with no offset, even if 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 |
Instance Variable Details |
unitsthe units part ofunit_string (i.e. 'days', 'hours',
'seconds').
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