def increment_month!
      if month != 12
        # just bump up a number
        self.date = ZDate.format_date(year_str, month + 1)
      else
        self.date = ZDate.format_date(year + 1)
      end
    end
def increment_month!
      if month != 12
        # just bump up a number
        self.date = ZDate.format_date(year_str, month + 1)
      else
        self.date = ZDate.format_date(year + 1)
      end
    end
Modifies self.
 bumps self to first day of next month