Returns the remaining time to live of a key that has a timeout. This
introspection capability allows a Redis client to check how many seconds a given
key will continue to be part of the dataset.

In Redis 2.6 or older the command returns `-1` if the key does not exist or if
the key exist but has no associated expire.

Starting with Redis 2.8 the return value in case of error changed:

- The command returns `-2` if the key does not exist.
- The command returns `-1` if the key exists but has no associated expire.

See also the `PTTL` command that returns the same information with milliseconds
resolution (Only available in Redis 2.6 or greater).

@return

@integer-reply: TTL in seconds, or a negative value in order to signal an error
(see the description above).

@examples

```cli
SET mykey "Hello"
EXPIRE mykey 10
TTL mykey
```