pandas.DatetimeIndex.argsort
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DatetimeIndex.argsort(*args, **kwargs)[source]
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Returns the indices that would sort the index and its underlying data. Returns: argsorted : numpy array See also 
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Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.20.3/generated/pandas.DatetimeIndex.argsort.html