How does my computer tell accurate time?

I have a question about my computer. I run Microsoft 98 and in the bottom right corner on the taskbar, there is a little clock. How does this thing keep the correct time? I turn my computer off every night, and when i turn it back on, it amazingly has the time right down to the second. How in the world does this work? When you turn off the power to the computer, doesn’t it cut off all electricity? I just don’t understand it. And thanks to anyone who answers my mundane question.

Think of your computer as a $1000 Timex. It has a battery in it. It is also used to remember power on passwords, etc.

There might be two, the main one to remember the system and a rechargable one to remember less important things like the time.

  • starfish is right about the battery
  • you’ll notice that it probably doesn’t keep the correct time; PC clocks are notorious for losing time.

I just checked mine (a mac), and it hit 7:01 right on Ma Bells beep. I don’t recall ever setting the clock.
Maybe my ISP sets it?

More likely Apple set it when they shipped it out to you. Since the battery has been in the computer from when it was assembled, it has kept time.

Maybe you’re in the same time zone as the plant where your computer was assembled? With Windows machines (at least those bought from Dell or other OEMs), you generally have to pick the time zone you’re in when you first set up your computer. But the manufacturer still sets the time correctly for you.

Computer clocks lose time when the battery starts going bad. Otherwise, they are relatively accurate. You can download software (try Dimension 4 at that sets your clock to an atomic clock, if you want accuracy.

If your clock starts losing time badly, get a new battery.

This page lists many programs which will synchronize your computer clock:

Check them out and pick one you like. Dimension 4 is good, but it doesn’t have a feature to synchronize the time upon command. Rather, it syncs immediately when started, then automatically every x hours as you specify. I prefer WorldTime, myself. NIST’s own Windows software is somewhat cumbersome to use, but it also does the job.

Please permit me a tiny hijack to ask where IS the battery in computers. I know it sounds silly, but on my ancient Amstrad, it was like the battery on a clock or similar applicance. Now I wouldn’t know where to find it, and I wonder what I will do when it needs to be replaced.

Every day I have to set the time on my computer, which is always exactly five minutes slow. It is part of a vast complex of computers that are evidently controlled somehow from downstairs. Why doesn’t it hold the time I set it at from day to day? I will try the suggested sites to set it and see if that works.

I forgot! My VCR at home always has the accurate time, even when there is a power failure that lasts a few hours! It also automatically gives the right time even when we change back and forth to daylight savings time! It probably does this by groping for a signal in the area. If a little VCR can do it why can’t a computer?

Today the battery is on the motherboard (the largeish circuit board that everything is plugged into). The battery is like a watch battery – thin, flat, silver and about the size of a nickel. Don’t go looking for AA or AAA batteries in there.

As for the Amstrad I have no idea. That is old enough that all bets are off when comparing to newer computers. In general these batteries have about a 5 year lifespan. Since people rarely keep the same PC for more than 5 years you don’t often hear of low batteries as being a problem.

This isn’t necessarily the case but it is possible that your computer is getting its time from a master time server on your computer network. The master time server is usually used to keep all of the servers on a network in synch but sometimes the administrators use it to update the desktop computers attached to the network as well.

Having everyone working on the same time (even if it is the wrong time as long as all computers agree on the wrong time) can be important when dealing with things such as databases. Computers like to know what order things happen in (it’s crucial in fact). If two computers disagree about the time they can start disagreeing about who did what first. In some cases this can mess things up royally.

Groping for a signal in the area is needlessly complex (and expensive) for a mere VCR. The answer is much simpler.

Like a computer many VCRs have a battery in them that keeps the time running and programs stored in case of a power failure. It takes very little power for this info to be retained so a small battery is quite sufficient.

As to daylight savings there is a simple little program that runs in the computer to tell it when daylight savings occurs as well as when it is in a leap year and so on. This is an algorithm and not a look-up table. If you search the internet you can find the formula that can be used for calculating dates.

You can see this on your computer. My Windows 95 computer at work can tell you the dates out to 2,099. I haven’t tried but I bet the Windows 2000 operating system could do even better.

My PlayStation2 came pre-programmed with the right time. It even kept the time after I unplugged it and took it to a friend’s house.

Ya know, when most guys hear the word ‘grope’, the word ‘signal’ is the least likely word to pop into their heads.


Anyway, here is a piece of software I use (For now. I plan to check some of the links above.) to keep time on my Win95 internet machine. It’s called SocketWatch, by Locutus Codeware, and it’s available right here: SocketWatch. The free version is all I use, and all I need, so I can safely say it’s functional freeware. Nota Bene: Because of how this program gets time (by contacting certain ports of various servers) it might not work behind a firewall. I think the help page addresses issues like this, so check it.