oracle 日期函数介绍:
在oracle中有很多关于日期的函数，如：
1、add_months()用于从一个日期值增加或减少一些月份
date_value:=add_months(date_value,number_of_months)
例：
SQL> select add_months(sysdate,12) "Next Year" from dual;

Next Year
----------
13-11月-04

SQL> select add_months(sysdate,112) "Last Year" from dual;

Last Year
----------
13-3月 -13

SQL>

2、current_date()返回当前会放时区中的当前日期
date_value:=current_date
SQL> column sessiontimezone for a15
SQL> select sessiontimezone,current_date from dual;

SESSIONTIMEZONE CURRENT_DA
--------------- ----------
+08:00 13-11月-03

SQL> alter session set time_zone='-11:00'
2 /

会话已更改。

SQL> select sessiontimezone,current_timestamp from dual;

SESSIONTIMEZONE CURRENT_TIMESTAMP
--------------- ------------------------------------
-11:00 12-11月-03 04.59.13.668000 下午 -11:
00

SQL>

3、current_timestamp()以timestamp with time zone数据类型返回当前会放时区中的当前日期
timestamp_with_time_zone_value:=current_timestamp([timestamp_precision])
SQL> column sessiontimezone for a15
SQL> column current_timestamp format a36
SQL> select sessiontimezone,current_timestamp from dual;

SESSIONTIMEZONE CURRENT_TIMESTAMP
--------------- ------------------------------------
+08:00 13-11月-03 11.56.28.160000 上午 +08:
00

SQL> alter session set time_zone='-11:00'
2 /

会话已更改。

SQL> select sessiontimezone,current_timestamp from dual;

SESSIONTIMEZONE CURRENT_TIMESTAMP
--------------- ------------------------------------
-11:00 12-11月-03 04.58.00.243000 下午 -11:
00

SQL>

4、dbtimezone()返回时区
varchar_value:=dbtimezone
SQL> select dbtimezone from dual;

DBTIME
------
-07:00

SQL>

5、extract()找出日期或间隔值的字段值
date_value:=extract(date_field from [datetime_value|interval_value])
SQL> select extract(month from sysdate) "This Month" from dual;

This Month
----------
11

SQL> select extract(year from add_months(sysdate,36)) "3 Years Out" from dual;

3 Years Out
-----------
2006

SQL>

6、last_day()返回包含了日期参数的月份的最后一天的日期
date_value:=last_day(date_value)
SQL> select last_day(date'2000-02-01') "Leap Yr?" from dual;

Leap Yr?
----------
29-2月 -00

SQL> select last_day(sysdate) "Last day of this month" from dual;

Last day o
----------
30-11月-03

SQL>

7、localtimestamp()返回会话中的日期和时间
timestamp_value:=localtimestamp
SQL> column localtimestamp format a28
SQL> select localtimestamp from dual;

LOCALTIMESTAMP
----------------------------
13-11月-03 12.09.15.433000
下午

SQL> select localtimestamp,current_timestamp from dual;

LOCALTIMESTAMP CURRENT_TIMESTAMP
---------------------------- ------------------------------------
13-11月-03 12.09.31.006000 13-11月-03 12.09.31.006000 下午 +08:
下午 00

根据日期计算第几周:

//计算第几周
public int GetWeekOfCurrDate(DateTime dt)
{
int Week = 1;
int nYear = dt.Year;
System.DateTime FirstDayInYear = new DateTime(nYear, 1, 1);
System.DateTime LastDayInYear = new DateTime(nYear, 12, 31);
int DaysOfYear = Convert.ToInt32(LastDayInYear.DayOfYear);
int WeekNow = Convert.ToInt32(FirstDayInYear.DayOfWeek) - 1;
if (WeekNow < 0) WeekNow = 6;
int DayAdd = 6 - WeekNow;
System.DateTime BeginDayOfWeek = new DateTime(nYear, 1, 1);
System.DateTime EndDayOfWeek = BeginDayOfWeek.AddDays(DayAdd);
Week = 2;
for (int i = DayAdd + 1; i <= DaysOfYear; i++)
{
BeginDayOfWeek = FirstDayInYear.AddDays(i);
if (i + 6 > DaysOfYear)
{
EndDayOfWeek = BeginDayOfWeek.AddDays(DaysOfYear - i - 1);
}
else
{
EndDayOfWeek = BeginDayOfWeek.AddDays(6);
}

if (dt.Month == EndDayOfWeek.Month && dt.Day <= EndDayOfWeek.Day)
{
break;
}
Week++;
i = i + 6;
}
return Week;
}

//本周是本年第几周
private int DatePart(System.DateTime dt)
{
int weeknow = Convert.ToInt32(dt.DayOfWeek);//今天星期几
int daydiff = (-1) * (weeknow+1);//今日与上周末的天数差
int days = System.DateTime.Now.AddDays(daydiff).DayOfYear;//上周末是本年第几天
int weeks = days/7;
if(days%7 != 0)
{
weeks++;
}
//此时，weeks为上周是本年的第几周