Проверьте Pd.Nat Python
In [34]: pandas.isnull(pandas.NaT)
Out[34]: True
In [35]: pandas.isnull(numpy.datetime64('NaT'))
Out[35]: True
FishBrawler
In [34]: pandas.isnull(pandas.NaT)
Out[34]: True
In [35]: pandas.isnull(numpy.datetime64('NaT'))
Out[35]: True