Real-time clock

A hardware clock that tracks calendar time independently of a computer’s main power state, often using a battery to keep running while the computer is off.

Quartz crystal oscillator: An electronic oscillator that uses a quartz crystal’s mechanical resonance to produce a stable frequency. Its steady oscillations provide the timing reference from which the clock counts seconds.
Time standard: A defined reference for measuring time consistently, such as Coordinated Universal Time. A real-time clock counts locally, while time standards define the reference its setting may approximate.
BIOS: Firmware that initializes computer hardware and begins the boot process on many personal computers. Traditional BIOS firmware reads clock settings during startup and may provide tools to change them.
Real-time operating system: An operating system designed to meet timing constraints with predictable response behavior. Despite the shared phrase, it concerns deadlines and scheduling, not calendar timekeeping hardware.
Coin-cell battery: A small, round battery commonly used to supply low-power devices for long periods. It powers the clock circuitry when the computer’s main supply is absent.
Unix time: A widely used representation of time as seconds elapsed since 00:00:00 UTC on 1 January 1970, excluding leap seconds. Operating systems often translate the clock’s calendar reading into this timestamp representation.
UEFI: A modern firmware interface that initializes hardware and loads an operating system. UEFI firmware can expose clock settings and use the hardware clock during boot.
Watchdog timer: A timer that resets or alerts a system if software fails to service it in time. It monitors system responsiveness rather than maintaining the current date and time.
Clock register: A hardware register that stores a value used to control or report device operation. Calendar fields such as seconds and hours are exposed through the clock’s registers.
System clock: The software-visible clock an operating system uses to measure and report current time. Unlike the battery-backed hardware clock, it typically runs from system timers while powered.

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Crystal oscillatorBroader topic: A low-power crystal oscillator commonly provides its one-second timing reference.