CentOS - Keeping Time with Chrony
Both CentOS-6 and CentOS-7 provide chrony as an alternative to the traditional ntp service. Chrony is a good fit for virtualised guests - a Vagrant box using the VirtualBox provider where you don't have/want the VirtualBox Guest Additions installed on the guest.
Installation
First install the chrony package with yum.
# yum -y install chrony
Virtual Guest Configuration
For a Virtual Guest where the host can be powered off, hibernated or put into sleep mode (i.e a laptop) such that the virtual guest is suspended and resumed frequently it's necessary to configure chrony to allow it to make step changes at any time. Usually, you only want to make step changes during the initial few updates to correct large time drifts and then small corrections are made gradually to stop sudden changes in system time affecting running applications/services.
With the following command step changes can be made by chrony whenever the system time is off by more than 1 second.
# sed -r -i \
-e 's~^(makestep )[0-9\.]+ (-)?[0-9]+$~\11 -1~' \
/etc/chrony.conf
Set chronyd to Run on Startup
CentOS-6
# chkconfig chronyd on
CentOS-7
# systemctl enable chronyd
Start the chronyd Service
CentOS-6
# service chronyd start
CentOS-7
Note: If chronyd was running before making changes to the configuration, run the following to make the changes reflect in timedatectl.
# systemctl restart systemd-timedated
The chronyd service can be started by toggling the timedatectl set-ntp command.
# timedatectl set-ntp no \
&& timedatectl set-ntp yes
Verification
CentOS-6
$ chronyc tracking
CentOS-7
$ chronyc tracking
And
$ timedatectl