Here is some info.
PC Real Time Clocks - Dependence on BIOS and Power
Every PC contains a Real Time Clock (RTC) implemented in the hardware. This clock runs continuously and provides time to the operating system clock when the computer is booted. The RTC runs off the same power source as the system BIOS.
The System BIOS has information saved in a little piece (64 bytes) of CMOS RAM . The CMOS power is supplied by a small battery, so its contents will not be lost after the PC is turned off. Therefore, there is a battery and a small RAM memory on board, which should never lose its information. The memory was in earlier times a part of the clock chip; now it's part of a highly integrated circuit . CMOS technology needs little power so the computer's battery is not in use much. Actually, there is not a battery on new boards, but an accumulator (Ni-Cad in most cases). It is recharged every time the computer is turned on. Some new motherboards have a technology called Dallas Nov-Ram which eliminates having an on-board battery as there is a 10 year lithium cell epoxyed directly in the chip.
If your BIOS is powered by external batteries, be sure that they are in good operating condition. Also, be sure that they do not leak - that may damage the motherboard and cause your BIOS to suddenly "forget" its configuration (and its time) and you may be looking for a problem elsewhere.
Clock on Taskbar and in Date/Time Tool Loses Time in Windows 95/98
Symptoms
The clock on the taskbar and in the Date/Time tool in Control Panel may experience a loss of time.
Cause
This problem can occur for either of the following reasons:
You change the year in the Date/Time tool. When you click a different year in the Date/Time Properties dialog box, the clock stops. When you click Apply or OK, the clock starts again, but it does not compensate for the length of time it was stopped.
You change the month or date in the Date/Time tool. When you click a different month or date in the Date/Time Properties dialog box, the current time is decreased by 5 to 10 seconds. Over time, this can result in a significant time loss. This problem does not affect the clock in your computer's CMOS.
Your computer battery is weak.
Your CMOS computer clock is losing time.
More information
To determine if the time loss is a result of a weak computer battery, follow these steps:
Click Start, point to Programs, and then click MS-DOS Prompt.
At the command prompt, type "time" (without quotation marks), and then press ENTER.
Compare this time with the time reported by the clock on the taskbar.
Type "exit" (without quotation marks), and then press ENTER.
If the computer's time and time on the clock are not the same, your computer's battery may be too weak to keep accurate time, and it should be replaced. For information about how to replace your computer's battery, refer to the documentation included with your computer.
Synopsis of Microsoft Knowledge Base Article Q189706 (August 24, 1998)