Synchronize System Clock with public NTP Servers on Debian 9 (Stretch)

Install:

apt-get install ntp

Show list of servers you are synching with (might take a few seconds for this list to populate right after the installation):

ntpq -p

If previous list was emtpy, try running:

dpkg-reconfigure ntp

NB: If your are running Linux containers (LXC) on your machine, you only need to install ntp on the host machine. The date and time in all containers will be identical with the date and time on the host.