How to keep Debian internal clock synchronized (with NTP servers)?

Debian expects you to install ntp yourself if you want your clock synchronized. Pretty much all you should have to do is apt-get install ntp. The default install, without any tasks, is fairly minimal.

I believe the GNOME desktop task, at least, will install it by default (as well as many other packages). Not sure if the other desktops will as well.

There isn't any other time synchronization method installed & running by default.

Note that with systemd apparently a different infrastructure involving timedatectl is in place (the old method still works though), see what the Arch guys write about it: https://wiki.archlinux.org/index.php/systemd-timesyncd