Clocks

How Your Computer Keeps Time When It Is Turned Off

Turn a computer off overnight and its clock is usually correct when you start it the next morning. It can even remember the date after spending much longer disconnected from normal power. That may seem surprising because the operating system was not running to count the passing seconds. The computer does not rely on the operating system for this job. It has a small hardware clock that can continue keeping time while most of the machine is off. After startup, the operating system reads that clock and can later compare its time with a trusted source over the internet.