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Merge pull request #15008 from jklymak/enh-add-variable-epoch
ENH: add variable epoch
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‎doc/api/api_changes_3.3/deprecations.rst

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@@ -491,3 +491,8 @@ experimental and may change in the future.
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``testing.compare.make_external_conversion_command``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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... is deprecated.
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`.epoch2num` and `.num2epoch` are deprecated
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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These are unused and can be easily reproduced by other date tools.
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`.get_epoch` will return Matplotlib's epoch.

‎doc/users/next_whats_new/2020-04-09-datetime-epoch-change.rst

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Dates now use a modern epoch
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----------------------------
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Matplotlib converts dates to days since an epoch using `.dates.date2num` (via
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`matplotlib.units`). Previously, an epoch of ``0000-12-31T00:00:00`` was used
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so that ``0001-01-01`` was converted to 1.0. An epoch so distant in the
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past meant that a modern date was not able to preserve microseconds because
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2000 years times the 2^(-52) resolution of a 64-bit float gives 14
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microseconds.
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Here we change the default epoch to the more reasonable UNIX default of
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``1970-01-01T00:00:00`` which for a modern date has 0.35 microsecond
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resolution. (Finer resolution is not possible because we rely on
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`datetime.datetime` for the date locators). Access to the epoch is provided
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by `~.dates.get_epoch`, and there is a new :rc:`date.epoch` rcParam. The user
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may also call `~.dates.set_epoch`, but it must be set *before* any date
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conversion or plotting is used.
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If you have data stored as ordinal floats in the old epoch, a simple
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conversion (using the new epoch) is::
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new_ordinal = old_ordinal + mdates.date2num(np.datetime64('0000-12-31'))

‎examples/ticks_and_spines/date_precision_and_epochs.py

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"""
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=========================
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Date Precision and Epochs
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=========================
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Matplotlib can handle `.datetime` objects and `numpy.datetime64` objects using
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a unit converter that recognizes these dates and converts them to floating
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point numbers.
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Before Matplotlib 3.3, the default for this conversion returns a float that was
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days since "0000-12-31T00:00:00". As of Matplotlib 3.3, the default is
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days from "1970-01-01T00:00:00". This allows more resolution for modern
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dates. "2020-01-01" with the old epoch converted to 730120, and a 64-bit
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floating point number has a resolution of 2^{-52}, or approximately
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14 microseconds, so microsecond precision was lost. With the new default
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epoch "2020-01-01" is 10957.0, so the achievable resolution is 0.21
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microseconds.
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"""
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import datetime
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import numpy as np
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import matplotlib
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import matplotlib.pyplot as plt
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import matplotlib.dates as mdates
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def _reset_epoch_for_tutorial():
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"""
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Users (and downstream libraries) should not use the private method of
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resetting the epoch.
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"""
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mdates._reset_epoch_test_example()
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#############################################################################
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# Datetime
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# --------
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#
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# Python `.datetime` objects have microsecond resolution, so with the
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# old default matplotlib dates could not round-trip full-resolution datetime
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# objects.
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old_epoch = '0000-12-31T00:00:00'
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new_epoch = '1970-01-01T00:00:00'
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_reset_epoch_for_tutorial() # Don't do this. Just for this tutorial.
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mdates.set_epoch(old_epoch) # old epoch (pre MPL 3.3)
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date1 = datetime.datetime(2000, 1, 1, 0, 10, 0, 12,
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tzinfo=datetime.timezone.utc)
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mdate1 = mdates.date2num(date1)
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print('Before Roundtrip: ', date1, 'Matplotlib date:', mdate1)
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date2 = mdates.num2date(mdate1)
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print('After Roundtrip: ', date2)
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#############################################################################
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# Note this is only a round-off error, and there is no problem for
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# dates closer to the old epoch:
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date1 = datetime.datetime(10, 1, 1, 0, 10, 0, 12,
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tzinfo=datetime.timezone.utc)
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mdate1 = mdates.date2num(date1)
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print('Before Roundtrip: ', date1, 'Matplotlib date:', mdate1)
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date2 = mdates.num2date(mdate1)
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print('After Roundtrip: ', date2)
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#############################################################################
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# If a user wants to use modern dates at microsecond precision, they
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# can change the epoch using `~.set_epoch`. However, the epoch has to be
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# set before any date operations to prevent confusion between different
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# epochs. Trying to change the epoch later will raise a `RuntimeError`.
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try:
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mdates.set_epoch(new_epoch) # this is the new MPL 3.3 default.
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except RuntimeError as e:
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print('RuntimeError:', str(e))
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#############################################################################
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# For this tutorial, we reset the sentinel using a private method, but users
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# should just set the epoch once, if at all.
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_reset_epoch_for_tutorial() # Just being done for this tutorial.
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mdates.set_epoch(new_epoch)
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date1 = datetime.datetime(2020, 1, 1, 0, 10, 0, 12,
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tzinfo=datetime.timezone.utc)
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mdate1 = mdates.date2num(date1)
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print('Before Roundtrip: ', date1, 'Matplotlib date:', mdate1)
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date2 = mdates.num2date(mdate1)
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print('After Roundtrip: ', date2)
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#############################################################################
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# datetime64
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# ----------
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#
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# `numpy.datetime64` objects have microsecond precision for a much larger
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# timespace than `.datetime` objects. However, currently Matplotlib time is
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# only converted back to datetime objects, which have microsecond resolution,
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# and years that only span 0000 to 9999.
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_reset_epoch_for_tutorial() # Don't do this. Just for this tutorial.
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mdates.set_epoch(new_epoch)
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date1 = np.datetime64('2000-01-01T00:10:00.000012')
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mdate1 = mdates.date2num(date1)
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print('Before Roundtrip: ', date1, 'Matplotlib date:', mdate1)
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date2 = mdates.num2date(mdate1)
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print('After Roundtrip: ', date2)
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#############################################################################
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# Plotting
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# --------
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#
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# This all of course has an effect on plotting. With the old default epoch
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# the times were rounded, leading to jumps in the data:
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_reset_epoch_for_tutorial() # Don't do this. Just for this tutorial.
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mdates.set_epoch(old_epoch)
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x = np.arange('2000-01-01T00:00:00.0', '2000-01-01T00:00:00.000100',
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dtype='datetime64[us]')
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y = np.arange(0, len(x))
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fig, ax = plt.subplots(constrained_layout=True)
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ax.plot(x, y)
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ax.set_title('Epoch: ' + mdates.get_epoch())
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plt.setp(ax.xaxis.get_majorticklabels(), rotation=40)
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plt.show()
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#############################################################################
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# For a more recent epoch, the plot is smooth:
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_reset_epoch_for_tutorial() # Don't do this. Just for this tutorial.
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mdates.set_epoch(new_epoch)
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fig, ax = plt.subplots(constrained_layout=True)
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ax.plot(x, y)
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ax.set_title('Epoch: ' + mdates.get_epoch())
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plt.setp(ax.xaxis.get_majorticklabels(), rotation=40)
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plt.show()
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_reset_epoch_for_tutorial() # Don't do this. Just for this tutorial.
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#############################################################################
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# ------------
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#
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# References
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# """"""""""
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#
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# The use of the following functions, methods and classes is shown
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# in this example:
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matplotlib.dates.num2date
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matplotlib.dates.date2num
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matplotlib.dates.set_epoch

‎lib/matplotlib/axes/_axes.py

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self.xaxis_date(tz)
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if ydate:
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self.yaxis_date(tz)
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ret = self.plot(x, y, fmt, **kwargs)
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self._request_autoscale_view()

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