There is something called a CMOS battery inside the computer on the motherboard. Its basically a very small car remote type battery, you know the flattened CR2032 like ones. Like flattened watch batteries. Anyhow, this battery keeps a timer awake. And it draws very less power to do that. Once this battery is dead, your computer wont be able to tell the time. This battery stays functional even when you switch off your computer.
CMOS battery on the motherboard that keeps the data on your BIOS chipset from losing power and resetting to factory defaults. Also there is a time keeping circuit that has a crystal inside it that is designed to oscillate at a specific frequency when specific voltage is applied to it. That is also how the CPU keep its timing as well. Without that crystal oscillator your whole system would be out of whack and nothing would work properly.
Not all computers do this.
I’ve tinkered with a Raspberry Pi 3, and it absolutely always needed to stay powered on or use an Internet service to know the time.
The difference between computers that can keep the time and ones that can’t is a “realtime clock” (RTC) circuit or chip.
An RTC is typically powered by a little coin cell battery. On a PC, this battery is usually also used to retain some of the computer’s basic hardware settings in its BIOS or UEFI.
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