select sysdate, systimestamp, current_date from dual;
select sysdate + 365 from dual;
select add_months(sysdate, -5) from dual;
select months_between(to_date('20180501'), sysdate) from dual;
select next_day(sysdate, '星期一') from dual;
select next_day(sysdate, 2) from dual;
select last_day(sysdate) from dual;
select trunc(sysdate,'MONTH') from dual;
select trunc(sysdate,'YEAR') from dual;
select extract(day from sysdate) from dual;
select round(to_date('20170916'),'MONTH') from dual;
select trunc(to_date('20170916'),'MONTH') from dual;