Have you ever wondered how computers can tell what time it is at all times ? Even if you keep it off, the next time you turn it on it will still show the correct time, but how do they do it? In this article we are going to explore how your PC always knows what time it is, and how it is able to remember it even when you turn it off.
Many of you will already know the answer to the second question: the motherboard has a small battery that allows, among other things, that the computer continues to count time and stay on time. However, that stack is no longer so important in modern PCs, since PCs know what time it is in other ways, which we will tell you next.
How does your PC know what time it is?
It is likely that when you bought a computer and turned it on for the first time, it will already show you the time correctly, even if you bought it in parts and assembled it yourself. This is because most modern operating systems – whether Windows, Linux or Mac OS – use the Internet to synchronize with a time server , which allows the PC to always know what time it is and adjust its clock accordingly.
For example, the Windows 10 clock uses the NTP Stratum 1 time server for Spain (if you live in another country, it is likely that you use another server but since they are all synchronized with each other, it is indifferent). These servers use cesium atomic clocks to keep track of time, and they are extremely accurate as they don’t use moving parts that can get out of sync, making them the most reliable around. Curiously, a unit of time is defined by 9,192,631,770 cycles of cesium , which means that these clocks use the same technology that scientists use to define a second in the first place.
In most modern computers this goes further, and is that even the BIOS is able to connect to the Internet and synchronize the time , so it knows it even before you install the operating system.
How does the PC keep track of the time even when it is off?
This is all very well, but let’s consider a specific scenario. Suppose you lose your internet connection, which means that your PC can no longer synchronize the time, so you get angry and turn off the PC. The next morning, you still have no Internet but when you turn on the PC, you check that it continues to show the time correctly; How is this possible? How can the PC still know what time it is?
This is because even without synchronization with the Internet time servers the PC is able to remember and keep track of the time by itself. In fact, you can see your PC’s ability to remember the time in the UEFI / BIOS, as this is loaded before starting the operating system.
Inside all computers there is an internal clock, and even when you turn off and disconnect the PC from the electrical network it will still be on time. This is because, as you already know, the motherboards have a small 3V battery that powers the CMOS chip, responsible for keeping the clock on -among other things- even when the rest of the PC has no power.
These batteries have a lifespan of around 10 years (so you will rarely have to change them), but they can still cause problems. If you receive a “CMOS error” when you start up the PC or the time is not displayed correctly, it may mean that it is running out of charge and that you will have to change it or even that the CMOS chip is damaged.
In any case, even if you did not have the battery on the motherboard (it would give you an error when starting) the PC would still know what time it is as long as you have it connected to the Internet, since it does not need it to synchronize with the Internet servers that let you know the time.
