Real-time Clocks

Almost everybody likes to be on time, but if you work with computers, time is really of the essence. The real-time clock is the small but important piece of hardware that keeps your computer up-to-date. Here, we explain the basics of the real-time clock and answer some of the most common questions about it.

Q: What is the RTC and why is it so important?
A: The real-time clock is a special computer chip on your system's motherboard that keeps time whether the computer is turned on or not. The RTC is responsible for providing date and time information to the system's Basic Input/Output (BIOS) chip, which then sends the time to the operating system. As a result, the date and time stamp on the files you change can ultimately be traced back to the RTC. Many systems built in the past few years have used the RTC for special alarm functions; for example, some computers use the RTC to "wake up" at a certain time every day. Power-management features that reduce power to components such as the monitor or diskette drive, also use the RTC to calculate idle time.

Q: When was the RTC introduced?
A: The first PC with a built-in RTC was the IBM PC/AT, which was introduced in 1984. It used a Motorola MC146818 complementary metal-oxide semiconductor (CMOS; a type of semiconductor that uses very little power) chip. Some people refer to the RTC as the CMOS chip.

Q: Does the RTC chip contain anything besides a clock?
A: The full name of the RTC chip is the real-time clock/nonvolatile random-access memory (RTC/NVRAM) chip because the clock chip contains 64 bytes of RAM.

Q: What powers the RTC when the computer is off?
A: A small battery attached to the motherboard keeps the nonvolatile RAM intact and the RTC ticking away. The battery may be soldered in place, be a removable lithium battery pack, or incorporate the RTC and battery into a single chip.

Q: Can I see the RTC in action?
A: One of the best ways to see the RTC working is to use RighTime's ViewCMOS clock-testing program, which you can download for free at http://www.rightime.com. This program lets you view the RTC, the BIOS representation of the RTC, and the operating system clock all keeping time.

Q: What can cause the RTC to lose time?
A: Actually, the minor losses in time (a minute or two) that you see when you look at your operating system's clock icon aren't caused by a problem with the RTC because the operating system clock is a separate clock. On the other hand, a major loss in time (becoming months or years behind) is often due to a dying battery that lacks the power to correctly run the RTC.

Q: How do I reset the RTC?
A: With very old systems using MS-DOS 3.2 and older, you had to use the BIOS configuration program and set the clock in the BIOS itself. With operating systems newer than MS-DOS 3.2 (including Windows), type date or time at a DOS prompt to set the clock. You can use a.m. or p.m. values with newer operating systems, but if you set the clock within the BIOS configuration, you must use the 24-hour clock (for example, where 1:00 p.m. is equal to 13:00)

Q: What is the role of the RTC in Y2K?
A: The RTC is at the root of the hardware part of the Year 2000 Problem. The problem results from using only two digits to represent the year. For example, in many systems, 99 would mean 1999, but 00 would mean 1900. The RTC helps cause the problem because most RTC chips lack a century field, storing dates as 12/11/99 instead of 12/11/ 1999.

Q: What provides the century information most RTCs lack?
A: The system BIOS chip must add the century information (19xx) and change it to 20xx when necessary.

Q: Can I supplement the RTC with an expansion card?
A: Yes you can. Some of the Y2K-solution expansion cards on the market contain a Y2K-compatible RTC that replaces the motherboard's clock.

Q: Do I need to replace my RTC?
A: Probably not. Most PC-compatible software gets its time from the BIOS representation of the RTC, not the RTC itself. So if you have a Y2K-compliant BIOS, you're all set. Some motherboards have clocks that have only a 100-year range (1979-2079), but this isn't an issue for most users. If your software queries the RTC directly for time, or if you need to set the RTC for a date of 2080 or beyond, you may have to replace the RTC.