Your computer does keep track of the time - in the BIOS. Windows reads the time from the system time. Other Operating Systems can get their time from a time server known as NTP.
If you've installed Linux, this can throw your time out. If the battery on your motherboard has gone flat or the motherboard has been damaged, this can also cause incorrect time but more than likely, some program has just altered it.
THE UPSHOT:
You can safely change it back without causing any trouble. You can even configure Windows to automatically set your time by periodically requesting time from a time server on the internet. Simply:
1) Right click on the clock in the bottom right;
2) Select Adjust Date/Time;
3) Click on the 'Internet Time' tab
4) Tick the Automatically synchronise with an Internet time server
5) Select a server or type your own in. There's a good chance your IPS provides one like ntp1.[your ISP name].com
6) Click the Update now button
Then, as long as you have an Internet connection, your time will always be right.
If you were running a UNIX server, I'd advise you to slew it back into time because UNIX/like machines don't take too kindly to huge jumps in time, especially if you're using the time for log entries, running cron jobs etc.