This question has been answered by BobRay. See the first response.
The default value for session_gc_maxlifetime is 1440. It's not a timestamp value, it's the minimum time between garbage collections (sort of) in seconds, so 24 minutes.
The session_cookie_lifetime is also in seconds (same default value), so 604800 sets it to one week.
Setting both to 1440 would give you 24-minute sessions. One day would be 86400 seconds.
It should clear out the older ones automatically, but I'd check anyway.