Let's suppose you are one of those people who likes to save energy and extend the lifespan of the computer by turning it off when it's not in use. If so, kudos to you!
The next time you start it up, you find it has remembered your settings and add-on devices. While much of this can be attributed to storing the information on your hard drive, what about dynamic information like the current date and time?
This Trade Secret lies buried in the computer's hardware. The BIOS, which detects and manages static and dynamic information about your hardware, is stored on a component called a Complementary Metal Oxide Semiconductor, or "CMOS" chip.
For the CMOS to remember and update information such as date and time, it must be kept alive by a small battery called the CMOS battery or system battery. Since the CMOS uses very little power, this battery can last two to three years. Therein begins a mysterious problem.
Unfortunately, people tend to discard their old computers around the same time that this battery is used up - likely prompted by the quirky effects of the dying battery - so while these effects may be often observed, they are seldom recognized. Because of this, documentation about the CMOS battery is rare.
Instead, you will find plenty of documentation about failures to detect hardware, and lost settings for the BIOS, hardware, and system time. Since much software depends on the system time for functionality, updates, and license validation, you will find lots of documentation on fixing an array of related, and very annoying problems.
The majority of such documentation offers solutions ranging from changing settings or installing and using diagnostic or corrective software or hardware, to extremes such as reinstalling or replacing the affected software or hardware, and even reformatting the hard drive and reinstalling the operating system and everything else!
Symptoms of a failing CMOS battery appear in an order which isn't very helpful - the most obvious symptoms tend to appear last. The most telling symptom is that the system clock is reset to the BIOS release date and time, or something else quite far from the current time. Next in certainty is that all BIOS settings have been reset to their defaults.
Other red flags are that the system has trouble recognizing existing hardware which it previously recognized without fail; or you discover some files have unrealistic dates; or some of your applications gripe at you to prematurely renew your license or even to register them again as if they were new.
We've heard this cures headaches too!