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I tried something.

I noted down the current time within my laptop (Dell, Windows 7). It was 12:15 PM 5 April 2014. And, I did the same with my mobile phone (Samsung Galaxy S2, Android Jelly Bean). It was 12:16 PM 5 April 2014.

Then, I switched off my mobile phone and my laptop for almost 10 hours and switch BOTH ON again. I saw the time was still synchronized and the time was perfect. How is this possible, whereas i was expecting the time to be delayed by 10 hours of current time?

And again, I tried the same with my mobile. And this time, I removed the battery for 2 hours, then it was asking me to enter the time again.

Can any one please tell be how this happens?

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For a computer, there is a battery, usually a watch style circular one. This powers a small piece of memory traditionally called CMOS. This memory stores the time and all the other BIOS settings. On a modern PC these batteries tend to last way past the computers useful life time.

A mobile phone/tablet isn't likely to have a secondary battery as they would take up too much space, but probably has a capacitor which can function as the same thing, albeit with a lot less capacity...hence the two hours before it forgot.

A PC motherboard CMOS battery

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