When the power goes out, you have reset the clocks on your stove, microwave, and other devices that plug into the wall. Yet, you do not have to do that with your phone or computer. The engineers who created the operating systems built in a way for the device to set the clocks themselves. For example, your phone will receive a signal from the carrier that allows the phone to connect to the network, but it also transmits the time. Find the Date and Time category in the settings to see if the time is set automatically. On Windows PCs, it is designed to periodically reach out and get the time from an Internet time server. Where do these sources get their times from?

Most internet time servers and cell networks get their times from atomic clocks.  These clocks are extremely accurate and do not suffer from drift. An atomic clock works by using the properties of atoms to keep time. The US Government pair of Atomic Clocks called NIST-F1 and NIST-F2 use cesium atoms. Microwaves will hit these atoms and as a result the electrons within them will get excited. These state changes happen billions of times a second. Cesium has a fix amount of state changes per atom totaling 9,192,631,770 times per second. This consistency makes atomic clocks extremely accurate. Cell networks pull their times from GPS satellites, which can have their own atomic clocks.

NIST-F1

With network latency and other interference, it is possible that your computer or phone may be off by a few milliseconds from the atomic clock, but overall the time will stay accurate.

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