def _timeseries(row):
    """
    Add timeseriesperiod to a dataframe 'row'.

    :param row: Pandas series for row of dataframe
    :return: returns amended row
    """
    period_data = str(row["period"])
    period_data = period_data[0:4] + "-" + period_data[4:6]
    row["timeseriesperiod"] = period_data
    return row