OK, I'm slowly understanding...

I run

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

time = pd.date_range('2000-01-01', periods=20)
depth = np.arange(20)
data = np.random.rand(20, 20)

fig, ax = plt.subplots()
ax.pcolormesh(time, depth, data)

For pandas-0.20 things run fine, and the units registry has keys:

dict_keys([<class 'datetime.date'>, <class 'datetime.datetime'>, 
<class 'pandas._libs.tslib.Timestamp'>, <class 'pandas._libs.period.Period'>, 
<class 'datetime.time'>, <class 'numpy.datetime64'>, <class 'str'>, <class 'numpy.str_'>, 
<class 'bytes'>, <class 'numpy.bytes_'>])

For pandas-0.21, we get the error, and the units registry only has keys:

dict_keys([<class 'str'>, <class 'numpy.str_'>, <class 'bytes'>, <class 'numpy.bytes_'>, 
<class 'datetime.date'>, <class 'datetime.datetime'>])

If I include

import pandas.plotting._converter as pandacnv
pandacnv.register()

then I get back the pandas stuff and the example runs fine.

dict_keys([<class 'str'>, <class 'numpy.str_'>, <class 'bytes'>, <class 'numpy.bytes_'>,
 <class 'datetime.date'>, <class 'datetime.datetime'>, <class 'pandas._libs.tslib.Timestamp'>, 
<class 'pandas._libs.period.Period'>, <class 'datetime.time'>, <class 'numpy.datetime64'>])

So, as @efiring said, what happened here was that pandas 0.21 moved the import of their registers out of the base import, and into their plotting area. More to the point, they now explicitly require the call of register().

FWIW, our pandas-0.21 plotting is indeed broken for other examples:

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

time = pd.date_range('2000-01-01', periods=20)
depth = np.arange(20)
data = np.random.rand(20, 20)

fig, ax = plt.subplots()
ax.plot(time, data[3,:])
plt.show()

... doesn't outright fail but gets:

Whereas it is decoded correctly in pandas-0.20.

Uh oh!

provide converters for datetime64 types #9610

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