[server] Help: Setting time and date (seriously)

Ok, I feel embarrassed. I have setup a number of Ubuntu machines since Ubuntu 7.04, and never came across this problem.

I am trying to set time and date on an Ubuntu machine. I noticed there is an issue after having setup an ntp server and asking my machines around to use it. It works well with all except one machine. That machine is 15s off. So after I struggeled with ntp, I fell back to the basics, I tried:

root@machine:/# date -s "2016-03-20 08:05:45"
Su 20 Mar 08:05:45 CET 2016
root@machine:/# date
Su 20 Mar 08:06:10 CET 2016

What could that be?

Thanks!