---
title: "When was the last leap second added?"
description: "A second added to the end of 31 December, 31 December 2016. What happened, why it has not happened since, and whether it plausibly could again."
url: https://atoneplace.net/the-last-time/leap-second/
site: "At One Place"
---

# When was the last leap second added?

**When was the last leap second added?: 31 December 2016**

A second added to the end of 31 December

Twenty-seven leap seconds have been inserted since 1972 to keep atomic time within 0.9 seconds of the Earth's rotation, which is irregular. In November 2022 the General Conference on Weights and Measures voted to stop the practice by 2035, because leap seconds break computer systems in ways that are difficult to test for — several major outages have been traced to one. A negative leap second, removing a second, has never been needed but is now plausible, since the Earth has recently been spinning slightly fast.

- **What:** A second added to the end of 31 December
- **When:** 31 December 2016
- **Category:** Science and measurement
- **Could this change?:** Yes. This streak could end, and if it does this page becomes a historical note rather than an answer.

## Science and measurement

| Question | When |
| --- | --- |
| When were the SI base units last redefined? | 20 May 2019 |
| When was an element last added to the periodic table? | 28 November 2016 |
| When did the number of planets last change? | 24 August 2006 |
| When was the metre last redefined? | 1983 |

> This page gives a date and never a "years ago". These pages are built as static files, so a computed interval would freeze at build time and quietly become wrong — which on a reference site is worse than making the reader do the subtraction.

## Common questions

### When was the last leap second added?

31 December 2016 — A second added to the end of 31 December.

### What happened?

Twenty-seven leap seconds have been inserted since 1972 to keep atomic time within 0.9 seconds of the Earth's rotation, which is irregular. In November 2022 the General Conference on Weights and Measures voted to stop the practice by 2035, because leap seconds break computer systems in ways that are difficult to test for — several major outages have been traced to one. A negative leap second, removing a second, has never been needed but is now plausible, since the Earth has recently been spinning slightly fast.

### Could it happen again?

Yes. This is one of the entries where the streak could plausibly end, which is stated on the page rather than left implied.

## Related pages

- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/)
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/)
- [When did the number of planets last change?](https://atoneplace.net/the-last-time/planet-count/)
- [When was the metre last redefined?](https://atoneplace.net/the-last-time/metre-redefined/)
- [When was the last Boeing 747 delivered?](https://atoneplace.net/the-last-time/boeing-747/)
- [When did Microsoft Office last change its default font?](https://atoneplace.net/the-last-time/office-default-font/)
- [When did the London Underground last gain a line?](https://atoneplace.net/the-last-time/london-underground-line/)
- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/)
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/)
- [When was the last VCR manufactured?](https://atoneplace.net/the-last-time/vcr/)
- [Everything, most recent first](https://atoneplace.net/the-last-time/)

## Sources

- The International System of Units (SI Brochure) — Bureau International des Poids et Mesures (https://www.bipm.org/en/publications/si-brochure)