In [1]: import matplotlib.pyplot as plt

In [2]: dates = np.array(['2000-01-01T00:00:00.000000000', '2000-01-02T00:00:00.000000000'], dtype='datetime64[ns]')

In [3]: values = np.arange(2)

In [4]: _, ax = plt.subplots()

In [5]: ax.plot(dates, values)
Out[5]: [<matplotlib.lines.Line2D at 0x7fba780ae130>]

In [6]: ax.set_xlim("1/1/1999", "1/1/2001")
Traceback (most recent call last):
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/axis.py", line 1736, in convert_units
    ret = self.converter.convert(x, self.units, self)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/category.py", line 57, in convert
    unit.update(values)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/category.py", line 216, in update
    _api.check_isinstance((str, bytes), value=val)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/_api/__init__.py", line 93, in check_isinstance
    raise TypeError(
TypeError: 'value' must be an instance of str or bytes, not a int

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 304, in process
    func(*args, **kwargs)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 2455, in _unit_change_handler
    line.recache_always()
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/lines.py", line 652, in recache_always
    self.recache(always=True)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/lines.py", line 656, in recache
    xconv = self.convert_xunits(self._xorig)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/artist.py", line 281, in convert_xunits
    return ax.xaxis.convert_units(x)
  File "/Users/matthewroeschke/opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/matplotlib/axis.py", line 1738, in convert_units
    raise munits.ConversionError('Failed to convert value(s) to axis '
matplotlib.units.ConversionError: Failed to convert value(s) to axis units: array(['2000-01-01T00:00:00.000000000', '2000-01-02T00:00:00.000000000'],
      dtype='datetime64[ns]')
Out[6]: (0.0, 1.0)

In [7]: import matplotlib; matplotlib.__version__
Out[7]: '3.7.0'