And you thought 2017 was a long year.

worker/ShutterstockIn 1972, Richard Nixon was president, Eugene Cernan became the last man to walk on the moon, and The Godfather won the Academy Award for Best Picture. But perhaps most notably, 1972 was the longest year in the history of recorded time, outlasting other years by a whopping two seconds.

So how’d that happen? To start, 1972 was a leap year, meaning it already had an extra 24 hours than most years. But on top of that, it also included two leap seconds: one added to the clock on June 30 and another added before 7 p.m. EST on December 31. Timekeepers around the world honored the leap seconds, and took action to adjust their clocks as part of an international agreement, as reported by the New York Times.

The reason for leap seconds is similar to the reason for leap years: they make it possible to align astronomical time, which is determined by Earth’s irregular rotation, with Coordinated Universal Time (UTC). Since the system was implemented in 1972, 27 leap seconds have been applied, the most recent on December 31, 2016.

For an example of how leap seconds are inserted, here’s what time looked like that New Year’s Eve:

2016 December 31, 23h 59m 59s

2016 December 31, 23h 59m 60s

2017 January 1, 0h 0m 0s

…instead of:

2016 December 31, 23h 59m 59s

2017 January 1, 0h 0m 0s

Currently, each leap second is usually announced about six months in advance by the International Earth Rotation and Reference Systems Service, which means you’ll have ample time to decide how to spend your extra second. For more can’t-miss trivia, read up on these 51 common “facts” that are actually false.