How does the Computer keeps Calendar?

As you know, even if the computer is shut down, it shows the date and time exactly when we boot it again. How this happens, even if the power is completely shut down? The magic is done by the CMOS (Complementary Metal Oxide Semi conductor) circuitry powered by a small 3 volt Lithium CMOS battery.

The computer has three separate clock systems.

1. Real Time Clock or RTC – This circuitry is present in the Mother board and is powered by the CMOS battery. Even if the AC power is disconnected, the battery will power this circuit to maintain the clock.
2. CMOS Clock – This is the “Logic Clock” stored in the “BIOS Chip” (Basic Input / Output System) which is the integrated circuit (IC) present in the Mother board. It keeps the memory of the settings without power from AC.
3. Operating System Clock or OS clock – It is the logic clock stored in the memory of the system and cleared each time the computer is On/ Off.
How the calendar is maintained?
When the computer boots, CMOS clock will synchronize with the Real Time Clock during POST (Power On Self Test) operation. When the computer boots, the RTC reads the current time and date from the CMOS clock and maintain its OS clock. The OS clock will not change unless the calendar is manually changed. When we manually change the time and date, RTC and OS clock will receive the current data from the CMOS clock and set.
The CMOS battery is a 3 volt Lithium cell ( NO. 2032) and is long lasting around 1-2 years. If it drains, the clock will not function. Most of the installed programmes and the websites like Google, Facebook etc will not work. This is because; the programmes are designed to work at the correct date and time. If the date and time changes due to error or improper shut down, these websites will show “SLS error”. We can correct this by manually adjusting the date and time.