[Nix-dev] Set date/time manually

[Eike]

Hello NixOS experts,

I want to test applications if they would work some time in the future
and so I thought to change the date using timedatectl. But it says that
it is not possible to use it on NixOS. The date command also doesn't
seem to work: the date/time is not changed after doing for example `sudo
date +%Y%m%d "20180210"`.

How can I temporarily set the system time some value?

Thanks for your advice
Eike