I need to do some testing that involves setting the clock into the future, and I can't see how to disable ntp syncing with timedatectl or anything else so I can change the clock and not have it sync to real time.
This is ubuntu 16.04
Just stop and disable both systemd-timesyncd
(installed by default) and ntp
(if you have installed it). Better take a backup before moving the system into the future, because interesting things may happen when you will bring it back into the past.
sudo systemctl stop systemd-timesyncd && sudo systemctl disable systemd-timesyncd
sudo systemctl stop ntp && sudo systemctl disable ntp
If the system is a virtual machine you must also make sure that it is not set to synchronize its time with the host.