On most windows computers there is a clock in the lower right hand corner. The time is maintained by the computer battery so that it won't lose time when your computer is off or unplugged. These batteries last 5 or so years and recharge every time you turn your computer on. If your clock is off it does not necessarily mean your battery is dying, it could be a sign of spyware or a virus (more on this problem in a later blog entry) or some other problem. Many times correcting the time once is all that is needed. Along with the clock is a calendar (it's hidden) that keeps track of the day, month and year.
To see what your computer is set at, just point at the clock with your mouse and double click. The date and time properties window will open. You can change the month, day, year, time and time zone easily from this window. Once finished click "apply" and verify all is correct (especially am/pm) then click "Ok" to close the window. Your clock in the corner should now present the proper time!
If the clock loses time while the computer is on (having not shut the computer off at all once the time was correct) there is a very good chance your system is infected with a bug or bugs of some kind. At the very least there are too many things running in memory. The battery has no control over the clock while the computer is on.
If your computer constantly loses track of the time (and the date) after having been turned off then the battery probably needs to be replaced. Replacing the battery sounds like a good subject for a different blog entry... preferably one that also covers the system BIOS, since that gets erased when the battery is removed. (This is not to be confused with your documents, pictures or programs, the BIOS controls the .... err, let's wait until the BIOS blog entry for all this). Your documents, programs and pictures are safe. (That is, as safe as anything can be on a computer... hmm, another good blog subject).
I would love to explain how to handle the clock on a Macintosh computer, but I really haven't a clue, so you'll have to google it or figure it out yourself (I haven't access to a Mac either to help you figure it out, sorry).