Keeping Us On Time

You wake up, hoping it’s earlier than you think. You grope groggily for your phone and bring it to life. The numbers on the screen tell you if it’s time to get up — or, if you’re lucky, that you can snooze a bit longer.

We count on our phones, smartwatches and computers to wake us up, keep us on schedule and help us make our appointments. But unlike the wristwatches of yore, we never set the clocks on our digital devices. So how do they always know the time?

Smartphones and other devices have internal electronic clocks that tick off seconds. But these clocks are not that precise and drift over time, so they are periodically corrected by signals from cellphone towers. Cell towers have GPS receivers that receive signals from GPS satellites, which in turn have onboard atomic clocks, and those clocks are steered by even better atomic clocks on the ground.

All told, GPS delivers time that is accurate to within 100 billionths of a second. So the time on our humble phones can be traced back, in just a few steps, to some of the best atomic clocks in the world.

Computer clocks also display atomic time, but they usually get it from a different route. NIST and other government institutions, as well as some large tech companies, run servers that receive time information directly from atomic clocks and provide it to devices upon request. NIST’s Internet Time Service, which operates around 20 time servers, receives more than a million hits per second, making it more popular than Google.

Learn more about how a few hundred clocks in government labs deliver accurate time to billions of devices around the world.

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