上官方网站看了一下expire的说明：
这样解释的：

The timeout will only be cleared by commands that delete or overwrite the contents of the key, including DEL, SET, GETSET and all the *STORE commands. This means that all the operations that conceptually alter the value stored at the key without replacing it with a new one will leave the timeout untouched. For instance, incrementing the value of a key with INCR, pushing a new value into a list with LPUSH, or altering the field value of a hash with HSET are all operations that will leave the timeout untouched.

如果用DEL, SET, GETSET会将key对应存储的值替换成新的，命令也会清除掉超时时间；如果list结构中添加一个数据或者改变hset数据的一个字段是不会清除超时时间的；如果想要通过set去覆盖值那就必须重新设置expire。

点击链接

EXPIREAT和EXPIRE 更新value都会重置过期时间。

set之前通过ttl获取到key的过期时间
set之后再把ttl的值给设成过期时间

192.168.1.9:6379> set key value
OK
192.168.1.9:6379> expire key 100
(integer) 1
192.168.1.9:6379> ttl key
(integer) 98
192.168.1.9:6379> set key value1
OK
192.168.1.9:6379> expire key 98
(integer) 1
192.168.1.9:6379> ttl key
(integer) 96
192.168.1.9:6379>

但这样子会有误差，不知道其他人有没有更好的方法。