Any PC, regardless of the operating system, always maintains it's own time on a hardware clock.
Unfortunately, those clocks are known to be inaccurate and -as you already pointed out yourself- the users
can alter the time/date.
If you want to get the universally correct time, you should consider setting up NTP to retrieve
the exact time from a world-wide server.
Likewise, once you got one machine that queries those servers, you can setup your other machines to contact your first machine, thus "propagating" the correct time accross your machines.
Have a look at: