A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit chip) that keeps track of the current time even when the computer is turned off. It is used in many kinds of computers, and is present in all modern personal computers. RTCs are also present in many embedded systems, though this should not be confused with real-time computing.
Real-time clocks run on a special battery that is not connected to the normal power supply. In contrast, clocks that are not real-time do not function when the computer is off. Do not confuse a computer's real-time clock with its CPU clock. The CPU clock regulates the execution of instructions.