def year_month_to_month_number(year, month, day=None):
 return ((((year - start_year) * 12) + (month - 1)) - start_month_0_indexed)

def year_month_to_month_number ( year , month , day = None ) : return ( ( ( ( year - start_year ) * 12 ) + ( month - 1 ) ) - start_month_0_indexed )
time periods are integers representing months in years .