Did you ever notice the computer keeping the accurate time records with utmost precision, and that too without any external interaction, when we turn the system off? And when you do turn it on back, it shows the same time.
There are quite many systems that are present in computers, laptops, mobiles, etc. that keeps on running on. Their power consumption is very less, even when we turn off the system. But the Clock system is one of those systems that will consume significant power. The iteration for the clock system will happen through when the system (computer, laptop, mobile, etc.) is switched off.
The reason for this is CMOS, which is an abbreviation for Complementary Metal-Oxide-Semiconductor. It is also many times referred to as CMOS RAM, NVRAM (known as Non-Volatile RAM) and RTC (Real-Time Clock). It is a specialized semiconductor chip, which is battery-powered, used to store various information and is situated inside the system. In layman terms, it can be referred to as a special kind of battery that is present inside the computer. The information that is stored can vary from simple time and date of the system to the hardware settings for the system. The most commonly used CMOS coin-cell battery that is used to get the CMOS memory powered up is Panasonic CR 2032 3V.
Usually, these CMOS batteries have an average life expectancy of around five years, although it highly depends on its usage and in what environment the computer resides. The depletion rate of the CMOS battery charge is linearly dependent on the power-off stage duration of the system. When the computer is shut down, the RAM, HDD, motherboard, processor, etc. are entirely turned off. It is this CMOS battery that keeps on running. So until this CMOS battery dies down, the computer will show the correct time, even when it is shut down for months and when it is without any internet access. But in practical use, the CMOS battery stores much more information than just date or time. Everything that the BIOS needs, is practically stored by the CMOS battery.
But if the CMOS battery fails, the computer would not be able to keep up with the correct date and date on the computer after it is shut down. For example, if you turn off the computer when the CMOS battery has died down, the date will be automatically set to January 1, 1990, and time will be set to 12:00 pm.