MySQL时间戳和时间格式转换函数：unix_timestamp and from_unixtime
unix_timestamp将时间转化成时间戳格式。
from_unixtime将时间戳转化成时间格式。
mysql> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
| 1251884321 |
+-----------------------+
1 row in set (0.00 sec)
mysql> select from_unixtime(1251884321);
+---------------------------+
| from_unixtime(1251884321) |
+---------------------------+
| 2009-09-02 17:38:41 |
+---------------------------+
1 row in set (0.00 sec)