About Setting the Time on All Nodes
Before starting the installation, ensure that the date and time settings on all the cluster 
nodes are set as closely as possible to  the same date and time. A cluster time 
synchronization mechanism ensures that the in ternal clocks of all the cluster members 
are synchronized. For Oracle RAC on Linux, you can use either the Network Time 
Protocol (NTP) or the Oracle Cluster Time Synchronization Service.
NTP is a protocol designed to synchronize the clocks of servers connected by a 
network. When using NTP, each server on the network runs client software to 
periodically make timing requests to one or more servers, referred to as reference NTP 
servers. The information returned by the timing request is used to adjust the server's 
clock. All the nodes in your cluster should use the same reference NTP server.

If you do not configure NTP, then Oracle configures and uses the Cluster Time 
Synchronization Service (CTSS). CTSS can also  be used to synchronize the internal 
clocks of all the members in the cluster.  CTSS keeps the member nodes of the cluster 
synchronized. CTSS designates the first node in the cluster as the master and then 
synchronizes all other nodes in the cluster to  have the same time as the master node. 
CTSS does not use any external clock for synchronization. 

Note: If you use NTP, then you must configure it using the  -x flag.
Note: Using NTP or CTSS does not protect your system against 
human error resulting from a change in the system time for a node.