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This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

2.45 Why is my PC's clock so inaccurate?

[From: rbean@execpc.com (Ron Bean)]

Well, you're not alone-- expensive workstations come with inaccurate clocks too! Usually they just run at the wrong speed, which means you can compensate with software that measures the drift rate and applies a correction factor. In the long run, this can be *very* accurate. Other programs can periodically set your clock to match another one that's known to be accurate (see the question on setting your clock).

If your clock is more erratic (eg, it stops when the machine is turned off, or the date gets scrambled), try replacing the battery (but remember to write down your CMOS settings first!). The CMOS RAM takes considerably less power than the clock, so it may keep working even though the battery is too weak to run the clock (see the question on replacing the battery for details).

The interrupt-based "DOS time" can also be affected by programs that disable interrupts for too long, so if you don't reboot your machine for a couple of days (and don't do anything else that resets the system time to match the CMOS clock) you may find that it has drifted also.

A typical cheap quartz watch is rated at +/- 15 sec per month (3 minutes per year) which is about 5.7 ppm (parts per million). In practice they are often much more accurate than that. Dallas Semiconductor rates their encapsulated clock modules for +/- 1 min per month, or 22.8 ppm. Many motherboards are off by 100 ppm.

To some extent this is because the manufacturer can't predict the operating temperature, which affects the crystal's frequency-- some machines run warmer than others, and some run more hours per day than others. The crystal's frequency will also change slightly over time as the crystal "ages". Clocks with external crystals can be "fine-tuned" with a trimmer capacitor, although I've never heard of anyone actually doing this on a motherboard.

The original IBM AT used the Motorola MC146818, which is a real-time clock plus 50 bytes of CMOS RAM. This chip is discussed in the book "The Undocumented PC", from Addison-Welsey. The Dallas Semiconductor DS1285 is a drop-in replacement for the MC146818, and the DS1287 is the same chip encapsulated with its own battery and clock crystal. Other variants include larger amounts of CMOS RAM.

 

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