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list_dates = [jan_1 jan_15 feb_1 feb_15 mar_1 mar_15 apr_1 apr_15 may_1 may_15 jun_1 jun_15 jul_1 jul_15 aug_1 aug_15 sep_1 sep_15 oct_1 oct_15 nov_1 nov_15 dec_1 dec_15]
# Here we are providing _year it will comes return that year in output.
pd.to_datetime(pd.Series(s) + '_2015', format='%b_%d_%Y')