Atomic Clocks are accurate to 1 second in 70 million years.

 Set Windows to synchronize your clock to a government atomic clock 
Did you know that Windows has a built-in feature to keep your computer's clock synchronized with atomic clock time? You heard correctly. This feature is found in Windows 2000, Windows XP, Windows Vista and probably all future versions of Windows.

How to make your computer clock self-correcting:

If you run Windows 2000 or newer, simply double-click on the clock in the lower right-hand corner of your screen.

This will bring up the Date and Time Properties window:

Click on the "Internet Time" tab and you will see this:

Type in the name of the name server as shown in the picture and then click "Update Now". If you are connected to the internet, your computer should now be synchronized to the very precise clock used by the US Navy. Note that you should also click on the "Time Zone" tab and make sure that your Time Zone is set correctly for your location and that "Daylight Savings Time" is checked. Once you have completed thes steps, click "OK" to close the dialog box. Windows will now keep your PC's clock synchronized to the atomic clock time. You will always know precisely what time it is. "You can set you watch by it" as the saying goes... haha...

What If I have Windows 98 or Millenium?

These version of Windows do not have the time synchronization feature. But you can use a free utility to accomplish the same thing. Try installing RocketTime, for instance. RocketTime has a list of many internet time servers built into it, so you typically do not have to change any settings at all. If the link to RocketTime does not bring you right to that program, just go to Download.com and type "RocketTime" in the "Search" field.