How Does a Computer Keep Time? A Simple Look at the CMOS

Computers perform so many tasks, but one of their simplest functions is displaying the time. If you have wondered how a computer keeps time, this is the perfect article for you to read.

A computer keeps time in the CMOS, within an IC on the motherboard. But that’s only the beginning, let’s dive a little deeper.

How a computer keeps time

Example of a BIOS

To fully understand how a computer keeps time, we need to get some basics understood first.

Although the time is kept in the CMOS, the CMOS is also responsible for retaining all the settings for the BIOS.

The BIOS (Basic Input/Output System) allows changes to settings involving the components on, and directly connected to the motherboard. The type of IC that contains the BIOS is usually a flash ROM device. It is programmable so updates can be written to it.

The CMOS previously was a separate IC, but in modern motherboards, it is usually integrated with the southbridge IC of the motherboard. A battery is connected to the IC containing the CMOS to keep it ‘alive’. This way, your computer can be unplugged from power, and it will retain your BIOS settings and the date and time.

By entering the BIOS when you power up your computer, the time and date can be set. When you save it, it gets updated in the CMOS.

The CMOS can only keep accurate time for a certain duration. Having an internet connection will ensure that the time gets kept far more accurately long term. This is achieved by Windows synchronizing the clock automatically with a server.

Related questions

Does BIOS time affect Windows time?

If you set the time in the BIOS and boot into Windows, the new time will update in the operating system. But other settings around the time won’t be compensated for, like the time zone for example.

By the same token, updating the clock settings in Windows applies the update to the CMOS as well.

Why do computer clock’s drift?

A CMOS relies on a crystal to keep the clock timing, which isn’t perfect. It will gradually start drifting from real-time. The crystal oscillates at a certain frequency by applying an electrical voltage to it. The oscillations from the crystal are used as a reference to keep time in the CMOS.


Although a computer’s time isn’t perfect, it still can keep itself on track if your computer is connected to the internet. Updation of the computer’s time is possible from multiple servers that you can choose from. They are selectable from the options under the clock settings.

So don’t expect your computer to keep perfect time when it is disconnected from the internet for long periods at a time. I’ve noticed that it doesn’t take long for the time to drift.