一、两个日期间隔天数，使用datediff()函数
例：
mysql> select datediff(now(),'20160401');
+----------------------------+
| datediff(now(),'20160401') |
+----------------------------+
|                          4 |
+----------------------------+
1 row in set (0.00 sec)

二、两个日期精确间隔，使用timediff
mysql> select timediff(now(),'2016-04-04 18:10:10');
+---------------------------------------+
| timediff(now(),'2016-04-04 18:10:10') |
+---------------------------------------+
| 15:41:28                              |
+---------------------------------------+
1 row in set (0.00 sec)