When calling expire or pexpire in redis, is time rounded up? - redis

Question

I am calling expire for an existing redis key. Let's say I am passing 5 for the value. If the key already exists and is 4.75 seconds away from expiring, does it stay at 4.75 seconds or is it rounded back up to 5 seconds?
I can use pexpire to get more granularity, but there is still a rounding problem with partial milliseconds - unless milliseconds is the smallest granularity in redis...
If it helps, here is my rate limit script, which takes a key, an amount to increment and a millisecond rate limit window, which keeps decrementing until the key drops out, at which point the next call adds the key and sets a fresh expire time. The new incremented value is then returned.
local f,k,a,b,c c=ARGV[2] f=redis.call k=KEYS[1] a=f('incrby',k,ARGV[1]) b=f('pttl',k) f('pexpire',k,math.min(b<0 and c or b,c)) return a
UPDATE
New rate limit script that does not have partial time issue, it only sets expire if the key does not have an expire set at all:
local f,k,a,b f=redis.call k=KEYS[1] a=f('incrby',k,ARGV[1]) b=f('pttl',k) if b<0 then f('pexpire',k,ARGV[2]) end return a

Answer 1

Does it stay at 4.75 seconds or is it rounded back up to 5 seconds?
It is back to full 5 seconds TTL.
unless milliseconds is the smallest granularity in redis...
It is milliseconds, for version 2.6 or greater
See Expire accuracy
In Redis 2.4 the expire might not be pin-point accurate, and it could
be between zero to one seconds out. Since Redis 2.6 the expire error
is from 0 to 1 milliseconds.
And
Keys expiring information is stored as absolute Unix timestamps (in milliseconds in case of Redis version 2.6 or greater).
You can play with some Lua scripts if you want to verify
EVAL "local result = {'Time at start', 0, 'Expires in (ms)', 0, 'Time at end', 0} \n result[2] = redis.call('TIME') \n redis.call('EXPIRE', KEYS[1], ARGV[1]) \n result[4] = redis.call('PTTL', KEYS[1]) \n result[6] = redis.call('TIME') \n return result" 1 myKey 5
Friendly view of the script:
local result = {'Time at start', 0, 'Expires in (ms)', 0, 'Time at end', 0}
result[2] = redis.call('TIME')
redis.call('EXPIRE', KEYS[1], ARGV[1])
result[4] = redis.call('PTTL', KEYS[1])
result[6] = redis.call('TIME')
return result