Where does SYSTIMESTAMP come from in multi-server Oracle RAC environment?
Up to 11g I know Oracle used the local OS time as the system time in a RAC environment, and I never heard that it changed so I'm comfortable saying it's still in place.  

You would sync all the servers together typically using NTP or another solution to ensure the time sync.  Usually if your servers local time is out of sync even by 30 seconds or so, you will tend to see other issues such as Kerberos auth issues so it's unlikely they are far off from each other.