Answer Description

The Real Time Clock (RTC) is a chip embedded in most electronic devices. The RTC keeps the time for a device and must have an alternative power source to use when the device is powered off. For computers this is the CMOS battery.

Wikipedia

A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time of day.