The computer clock will gradually get out of line with reality since it's not a very accurate beast. Some software can check the time with an outside source on the network or Internet and regularly adjust your computer by a few seconds so it never drifts so far off that you would notice.
If your computer is a part of a domain (not possible in Home Edition) you do not need to do anything. The computer will automatically update it's time by querying the PDC Emulator (usually the first DC n the domain).
If you still want to adjust your clock (you shouldn't because it might have adverse effects on some of your network connections and management tools) you can run the following command:
(%logonserver% can be used as a variable, but you can specify a DC name if you want).
If you're not a part of a domain and still want to update your clock automatically, you can right click the clock found on the tray area on the lower right of your screen and choose "adjust time/date".
Click on the Internet Time tab, then select the "Automatically synchronize with an Internet time server" checkbox.