GBase 8c 数据类型-日期时间格式

支持日期格式如下：

GBase 8c：
  SELECT add_months(date '2005-05-31',1);  -- > 2005-06-30

  SELECT last_day(date '2005-05-24');      -- > 2005-05-31

  SELECT next_day(date '2005-05-24', 'monday'); -- > 2005-05-30

  SELECT next_day(date '2005-05-24', 2); -- > 2005-05-30

  SELECT months_between(date '1995-02-02', date '1995-01-01'); -- > 1.0322580645161

  SELECT trunc(date '2005-07-12', 'iw');   -- > 2005-07-11

  SELECT round(date '2005-07-12', 'yyyy'); -- > 2006-01-01