In order to measure elapsed time in a system, a standard unit of measure and an instrument to count the units is required. This instrument is called a clock. A Real Time Clock (RTC) is a clock which is incorporated in an electronic product that keeps track of time, even when the rest of the system is switched off. An RTC is based on a quartz oscillator running at 32.768 kHz. With an RTC, it is easy to derive a 1Hz signal (or a multiple of) and keep track of seconds, minutes, hours, days, etc.