volume value is based on the pub date year
    pub_date is a python time object

def calculate_journal_volume(pub_date, year):
    """
    volume value is based on the pub date year
    pub_date is a python time object
    """
    try:
        volume = str(pub_date.tm_year - year + 1)
    except TypeError:
        volume = None
    except AttributeError:
        volume = None
    return volume