You have two options for time synchronization:

1) NTP : an operating system configured network time protocol (NTP),

To Deconfigure NTP services on both nodes

/sbin/service ntpd stop
chkconfig ntpd off
mv /etc/ntp.conf /etc/ntp.conf.original
rm /var/run/

To Setup NTP , Refer to :

2) Oracle Cluster Time Synchronization Service.
Oracle Cluster Time Synchronization Service is designed for organizations whose cluster servers are unable to access NTP services.
If you use NTP, then the Oracle Cluster Time Synchronization daemon (ctssd) starts up in observer mode.

If you do not have NTP daemons, then ctssd starts up in active mode and synchronizes time among cluster members without contacting an external time server.

To confirm that ctssd is active after installation, enter the following command as the Grid installation owner:
$ crsctl check ctss