Like `TTL` this command returns the remaining time to live of a key that has an
expire set, with the sole difference that `TTL` returns the amount of remaining
time in seconds while `PTTL` returns it in milliseconds.

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.

@return

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

@examples

```cli
SET mykey "Hello"
EXPIRE mykey 1
PTTL mykey
```