Issue description

It's impossible to set time and date via timedatectl. @abbradar said that it's a bug.

Steps to reproduce

$ timedatectl set-time "2016-01-01 00:00:00"
Failed to set time: Changing system settings via systemd is not supported on NixOS.
(It's reproducible for any time and date)

Technical details

• System: NixOS (with nixpkgs from git)
• Nix version: 1.11.2
• Nixpkgs version: 16.09pre78848.9549257