On Tuesday, the world will receive a gift of time: a single, extra second known as a “leap second.”

At that moment, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23h 59m 59s, followed by the leap second 23h 59m 60s. July 1 will continue as usual, beginning with 0h 0m 0s.

Leap years come like clockwork every fourth calendar year, but leap seconds are less predictable. The June 30, 2015, leap second is the 26th time an extra second has been tacked on to a day since atomic clocks began governing our time in 1967.

The June 30 leap second was decreed in January by astronomers at the International Earth Rotation and Reference Systems Service in Paris, who measure Earth’s rotation and compare it with the time kept by atomic clocks.

The leap second is designed to keep astronomical time in sync with atomic time.

The astronomers at the International Earth Rotation and Reference Systems Service keep track of the difference between astronomical time and atomic time. They call for a leap second to be added whenever Earth’s observed position is on track to be out of sync with UTC time by more than 0.9 of a second. Leap seconds traditionally are added on June 30 or Dec. 31.


Only subscribers are eligible to post comments. Please subscribe or login first for digital access. Here’s why.

Use the form below to reset your password. When you've submitted your account email, we will send an email with a reset code.