A half-century ago, the world’s scientists agreed to stop basing time on astronomy. Instead, they defined the building block of time — one second — based on the cesium atom. The 13th General Conference on Weights and Measures in October 1967 defined the “second” as the vibration (9,192,631,770 periods of radiation) of the cesium atom, according to the National Institute of Standards and Technology (NIST).

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But since the world slows by about 2 milliseconds a day, a leap second needs to be added about every 500 days. Civil time must be adjusted so that the world’s atomic clocks do not vary from the Earth’s rotational time by more than 0.9 seconds, according to the observatory.

Said Geoff Chester: “By adding a leap second, we’re essentially stopping our atomic clocks to let the Earth catch up.”

To synchronize the world, timekeepers will add one second to the clocks on Dec. 31. Under the concept of Coordinated Universal Time (UTC), a leap second will be added at 23 hours, 59 minutes and 60 seconds — or 6:59:60 p.m. Eastern Time. The second will be added to the U.S. Naval Observatory’s Master Clock facility, according to Chester, an astronomer at the observatory.

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Since 1972, when the first leap second occurred, 26 tics have been added. The last was June 30, 2015.

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Time is of the essence and it affects us all. For example, to ensure that all of Google’s applications — like Gmail, YouTube and Google Maps — communicate with each other, perfect time is needed. During this leap second, Google will “smear” time to provide a seamless transition.

Google has “application programming interfaces” that integrate its services. Instead of applying leap seconds to the company’s servers, Google evenly distributes that one second over a 20-hour period.