[root@cent01 run]# cat /etc/sysconfig/ntpd
# Drop root to id 'ntp:ntp' by default.
#OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid"

# Set to 'yes' to sync hw clock after successful ntpdate
SYNC_HWCLOCK=no

# Additional options for ntpdate
NTPDATE_OPTIONS=""

[root@cent01 run]# cat /etc/ntp.conf
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery

# Permit all access over the loopback interface.  This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
restrict -6 ::1

# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

#broadcast 192.168.1.255 key 42         # broadcast server
#broadcastclient                        # broadcast client
#broadcast 224.0.1.1 key 42             # multicast server
#multicastclient 224.0.1.1              # multicast client
#manycastserver 239.255.254.254         # manycast server
#manycastclient 239.255.254.254 key 42  # manycast client

# Undisciplined Local Clock. This is a fake driver intended for backup
# and when no outside source of synchronized time is available.
server 127.127.1.0      # local clock
fudge  127.127.1.0 stratum 10

# Drift file.  Put this in a directory which the daemon can write to.
# No symbolic links allowed, either, since the daemon updates the file
# by creating a temporary in the same directory and then rename()'ing
# it to the file.
driftfile /var/lib/ntp/drift

# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography.
keys /etc/ntp/keys

# Specify the key identifiers which are trusted.
#trustedkey 4 8 42

# Specify the key identifier to use with the ntpdc utility.
#requestkey 8

# Specify the key identifier to use with the ntpq utility.
#controlkey 8

[root@cent01 run]# cat /etc/ntp/step-tickers
0.us.pool.ntp.org
1.us.pool.ntp.org
2.us.pool.ntp.org
3.us.pool.ntp.org

Time synchronization
     With the Guest Additions installed, VirtualBox can ensure that the
     guest’s system time is better synchronized with that of the host.

     For various reasons, the time in the guest might run at a slightly different rate than the
     time on the host. The host could be receiving updates via NTP and its own time might not
     run linearly. A VM could also be paused, which stops the flow of time in the guest for a
     shorter or longer period of time. When the wall clock time between the guest and host only
     differs slightly, the time synchronization service attempts to gradually and smoothly adjust
     the guest time in small increments to either “catch up” or “lose” time. When the difference
     is too great (e.g., a VM paused for hours or restored from saved state), the guest time is
     changed immediately, without a gradual adjustment.

     The Guest Additions will re-synchronize the time regularly. See chapter 9.12.3, Tuning
     the Guest Additions time synchronization parameters, page 141 for how to configure the
     parameters of the time synchronization mechanism.