pandas.Series.to_period
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Series.to_period(freq=None, copy=True)[source]
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Convert Series from DatetimeIndex to PeriodIndex with desired frequency (inferred from index if not passed) Parameters: freq : string, default Returns: ts : Series with PeriodIndex 
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Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.20.3/generated/pandas.Series.to_period.html