def METHOD_NAME(data):
    if isinstance(data, listOfDataFrames):
        start_date = min([dataitem.index[0] for dataitem in data])
        end_date = max([dataitem.index[-1] for dataitem in data])
    else:
        start_date = data.index[0]
        end_date = data.index[-1]
    return start_date, end_date