Even after rebooting your computer you get exact date and time. Have you ever given it a thought that how is it possible? The answer is pretty simple; there must be an independent power source running a clock which runs even when system main power is off. Let us explore it.
RTC- Real Time Clocks
There are 2 clocks in a computer. One is a hardware clock known as the Real Time Clock and the other is Software Clock.
Real Time Clock is battery backup power clocks so that it tracks the time even while the computer is turned off, or in low power state. Basically RTC is not a physical clock but is an IC which is present on the motherboard and responsible for timing functioning of the system and system clock. Real Time Clock is responsible to make sure that all the processes occurring in the system are properly synchronized (basically this is task of system clock, but system clock is dependent on RTC, therefore RTC is indirectly responsible for interrupts, timer, task scheduling and synchronization etc.). Today many companies like Philips, ST Microelectronics, Texas Instruments manufacture RTCs. There has been a continuous development in RTC, like lowering power consumption, improving frequency stability.
System Clock is maintained by the kernel of an operating system and is used to set the tasks and processes - their synchronization and scheduling, settings and managing interrupts, setting timer etc. The system clock reports seconds and microseconds since a start point from the system boot up procedure. Basically the system clock is digital signal emitter, which emits signal composed of high (1) and low (0), because all the machines and their processes understand the language of binary.