def life_time(data_frame):
    """
    Calculates the life time of each row. Defined as 'dateModified' -
    'dateCreated'.

    :data_frame Pandas data frame holding the dateModified and dateCreated
        columns
    :return Life time series
    """
    return data_frame.assign(
            lifeTime=lambda x: x.dateModified - x.dateCreated)['lifeTime']