How does my computer mark time when it is switched off?

This may sound a stupid question to all you computer boffins out there and I am sure there’s an obvious answer, but call me stupid, I cannot work it out.

I hadn’t used my old computer in a while and for about a year it was sitting in a box in the corner of the room. When I re-connected it it came up with the correct date and time. So I am wondering, how does it know how long it has been out of use. If I take the batteries out of my alarmclock it just stops, so how the the computer work it out without electricity?

The Puzzled Pookah

Because you didn’t take the battery out.

Your computer (and all other PC type computers I’m familiar with) have a small battery connected to the motherboard that keeps essential settings from being lost, and powers the clock. These batteries are usually good for 3 to 5 years before they need to be replaced. Yours seems to be in good shape.


> If I take the batteries out of my alarmclock
> it just stops, so how the the computer work
> it out without electricity?

Because there is a battery in your computer (ta da). For literally years (since the Regan Era) PCs have had a small battery put on thier motherboard that is used to keep the internal clock accurate. And thus, over time, these batteries do fail, and need to be replaced. Since this battery is only used for time though it draws very little power, lasting at least as long as a typical watch battery (which is what? 2 to 4 years?)

Well, a lithium battery of some sort.
Some are soldered in, others are removable. I’ve see both watch type “coin” batteries and others that look like a short AA. Typically these last ~5 years

It powers the clock and probably some some RAM to keep some of your settings set when the machine is off.

On Macs, if your clock keeps saying that it is 1950 and your settings are getting wonky, its a good sign that the battery is dying.


If this battery dies, do I lose ability to boot up? Does this have something to do with BIOS? Is it critical to replace this battery is my clock keeps losing time? And, where should I look for it on the motherboard?

I know not what I speak of here…

  • Jinx

Losing the battery / bad battery:

If this happens, every time you boot up, your system time will be reset to 0 (1/1/1970 I think). It will also lose your bios settings, which will cause them to be set to the “burned-in” defaults, which could cause a problem, but is usually ok, since the default is to auto-detect your drives and auto-configure your interrupts.

Most times this just produces a warning message which requires you to press F1 to continue boot up.

I had a machine that did this, and I even replaced the battery twice. I think there was a problem with the battery->motherboard connection.