一、获取系统时间函数
1.1 获取当前完整时间
select now();
david=# select now();
now
-------------------------------
2013-04-12 15:39:40.399711+08
(1 row) david=#
current_timestamp 同 now() 函数等效。
david=# select current_timestamp;
now
-------------------------------
2013-04-12 15:40:22.398709+08
(1 row) david=#
1.2 获取当前日期
select current_date;
david=# select current_date;
date
------------
2013-04-12
(1 row) david=#
1.3 获取当前时间
select current_time;
david=# select current_time;
timetz
--------------------
15:43:31.101726+08
(1 row) david=#
二、时间的计算
david=# select now();
now
-------------------------------
2013-04-12 15:47:13.244721+08
(1 row) david=#
2.1 两年后
david=# select now() + interval '2 years';
?column?
-------------------------------
2015-04-12 15:49:03.168851+08
(1 row) david=# select now() + interval '2 year';
?column?
-------------------------------
2015-04-12 15:49:12.378727+08
(1 row) david=# select now() + interval '2 y';
?column?
------------------------------
2015-04-12 15:49:25.46986+08
(1 row) david=# select now() + interval '2 Y';
?column?
-------------------------------
2015-04-12 15:49:28.410853+08
(1 row) david=# select now() + interval '2Y';
?column?
-------------------------------
2015-04-12 15:49:31.122831+08
(1 row) david=#
2.2 一个月后
david=# select now() + interval '1 month';
?column?
------------------------------
2013-05-12 15:51:22.24373+08
(1 row) david=# select now() + interval 'one month';
ERROR: invalid input syntax for type interval: "one month"
LINE 1: select now() + interval 'one month';
^
david=#
2.3 三周前
david=# select now() - interval '3 week';
?column?
-------------------------------
2013-03-22 16:00:04.203735+08
(1 row) david=#
2.4 十分钟后
david=# select now() + '10 min';
?column?
-------------------------------
2013-04-12 16:12:47.445744+08
(1 row) david=#
说明：
interval 可以不写，其值可以是：
| Abbreviation | Meaning |
| Y | Years |
| M | Months (in the date part) |
| W | Weeks |
| D | Days |
| H | Hours |
| M | Minutes (in the time part) |
| S | Seconds |
2.5 计算两个时间差
使用 age(timestamp, timestamp)
david=# select age(now(), timestamp '1989-02-05');
age
----------------------------------------
24 years 2 mons 7 days 17:05:49.119848
(1 row) david=#
david=# select age(timestamp '2007-09-15');
age
------------------------
5 years 6 mons 27 days
(1 row) david=#
三、时间字段的截取
在开发过程中，经常要取日期的年，月，日，小时等值，PostgreSQL 提供一个非常便利的EXTRACT函数。
EXTRACT(field FROM source)
field 表示取的时间对象，source 表示取的日期来源，类型为 timestamp、time 或 interval。
3.1 取年份
david=# select extract(year from now());
date_part
-----------
2013
(1 row) david=#
3.2 取月份
david=# select extract(month from now());
date_part
-----------
4
(1 row) david=#
david=# select extract(day from timestamp '2013-04-13');
date_part
-----------
13
(1 row) david=#
david=# SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute');
date_part
-----------
40
(1 row) david=#
3.3 查看今天是一年中的第几天
david=# select extract(doy from now());
date_part
-----------
102
(1 row) david=#
3.4 查看现在距1970-01-01 00:00:00 UTC 的秒数
david=# select extract(epoch from now());
date_part
------------------
1365755907.94474
(1 row) david=#
3.5 把epoch 值转换回时间戳
david=# SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 1369755555 * INTERVAL '1 second';
?column?
------------------------
2013-05-28 23:39:15+08
(1 row) david=#
以上是基本的PG时间/日期函数使用，可满足一般的开发运维应用。
3.6 求取两个timestamp之间的月份差
select
EXTRACT(YEAR from age('2016-09-19 00:00:00.000000', '2012-12-31 00:00:00.000000'))*12+
EXTRACT(MONTH from age('2016-09-19 00:00:00.000000', '2012-12-31 00:00:00.000000'))
详细用法请参考：
PostgreSQL官方说明：http://www.postgresql.org/docs/9.2/static/functions-datetime.html
