What are leap seconds?

Coordinated Universal Time (UTC) is a time standard based on two other standards, International Atomic Time (TAI) and Universal Time (UT1). It aims at being at a whole second offset from TAI, while keeping UTC and UT1 within 0.9 seconds of each other.

In order to accomplish that, UTC bases itself on TAI, and gets leap seconds added to it when considered necessary by the International Earth Rotation Service (IERS), in a semi-annually published bulletin called Bulletin C which announces whether or not a leap second is inserted in June 30th and/or December 31st, meaning the UTC clock may reach 23:59:60 on these dates.

Note

With timezones, the leap second may not be inserted at 23:59, but at another time. For example:

• In France, using Central European Time (CET, UTC+01:00), the leap second was inserted on January 1st, 2017, at 00:59:60.

• In Australia, using Australian Western Central Standard Time (AWCST, UTC+08:45), the leap second was inserted on January 1st, 2017, at 08:44:60.

• In the United States, using Mountain Time Zone (UTC-07:00), the leap second was inserted on December 31st, 2016, at 16:59:60.

For more information, you can read The Unix leap second mess (madore.org), as well as the Wikipedia pages linked above.