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How does my cellphone know what time it is?
The other day I was sitting at dinner with a group of people, and I asked of no one in particular, "What time is it?" Two people looked at their cellphones and gave me two different answers: 6:57 and 6:58. If the person who answered 6:57 had answered first, I would have thought I had caught them just as the time was changing, but it was the 6:58 person who answered first. So it seems their cellphones were receiving two different signals. They turned out to use different carriers. Until this happened, I just assumed cellphones use GPS signals to set the time, or that the cell towers used GPS to get the time, which they then transmit to the phones. But this does not seem to be the case.
So what's the scoop? Do different phone companies use different timebases? Do any of them use GPS time, or do they use atomic time?
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