pandas.DatetimeIndex.to_perioddelta
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DatetimeIndex.to_perioddelta(freq)[source]
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Calcuates TimedeltaIndex of difference between index values and index converted to PeriodIndex at specified freq. Used for vectorized offsets New in version 0.17.0. Parameters: freq : Period frequency Returns: y : TimedeltaIndex 
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Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.20.3/generated/pandas.DatetimeIndex.to_perioddelta.html