This may be well known among the geeks who have several different operating systems on their computer and can boot up to any one they select but each operating system (at least with Windows) keeps its own track of the overnight time change.
So if you started up your computer this morning, booted into Windows 7 and found the time had correctly jumped forward that's the good news ... but if you then restarted the computer and booted up into say Windows 8 Preview you would find that it had jumped forward an hour too ... not from last night's time but from this morning's time already changed by Windows 7 ..... so you have to set it back an hour.
And if you have another Windows on there .....
But once the first boot into the operating system has taken place it stays stable .... until the next time change.
Since having the wrong system time can affect the way some applications work it's more than just knowing what time it is .....