The chronological change spotlights some of the quirks of an increasingly critical task—keeping the world's clocks perfectly synced.

(Related: "Leap Year: How the World Makes Up for Lost Time".)

Coordinated universal time (UTC) is an atomic time scale derived from a variation of the metallic element cesium's atom. This truly atomic "clock" ticks with microwave light about nine billion times each second, allowing us to slice and dice time with extreme precision.

The official UTC time is set by the Paris-based International Bureau of Weights and Measures, which gathers contributions from labs in some 50 nations and computes an internationally agreed-on average.

This year's leap second—the 26th to be added to UTC since 1972—exists because time was traditionally based on a full