1, now() 返回当前日期和时间
mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2015-03-18 22:45:17 |
+---------------------+
1 row in set (0.00 sec)
2.curdate() 返回当前日期
mysql> select curdate();
+------------+
| curdate() |
+------------+
| 2015-03-18 |
+------------+
1 row in set (0.00 sec)
3.curtime() 返回当时时间
mysql> select curtime()
-> ;
+-----------+
| curtime() |
+-----------+
| 22:48:00 |
+-----------+
1 row in set (0.00 sec)
4.date_add() 当前日期加N天（月，年）
mysql> select date_add(curdate(), interval 2 day);
+-------------------------------------+
| date_add(curdate(), interval 2 day) |
+-------------------------------------+
| 2015-03-20 |
+-------------------------------------+
1 row in set (0.00 sec)
5, date_sub 与date_add相反，当前日期减N天（月，年）
mysql> select date_sub(curdate(), interval 1 day);
+-------------------------------------+
| date_sub(curdate(), interval 1 day) |
+-------------------------------------+
| 2015-03-17 |
+-------------------------------------+
1 row in set (0.00 sec)
6,datediff 返回两个日期间隔的天数
mysql> select datediff('2015-03-18', '2015-03-13');
+--------------------------------------+
| datediff('2015-03-18', '2015-03-13') |
+--------------------------------------+
| 5 |
+--------------------------------------+
1 row in set (0.05 sec)