The Earth’s rotation and speed changes with the weather and geological events. This means atomic time and solar time may not be identical. Sometimes leap seconds are needed for an adjustment. On 31 Dec 2016, a leap second was added, and the time 23:59:60 existed.

Leap second

Screenshot of the UTC clock from time.gov during the leap second on December 31, 2016. In the U.S., the leap second took place at 18:59:60 local time on the East Coast, at 15:59:60 local time on the West Coast, and at 13:59:60 local time in Hawaii.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise observed solar time (known as UT1 and which varies due to irregularities and long-term slowdown in the Earth’s rotation). The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses precise atomic time and consequently would run … Continue Reading (18 minute read)

6 thoughts on “The Earth’s rotation and speed changes with the weather and geological events. This means atomic time and solar time may not be identical. Sometimes leap seconds are needed for an adjustment. On 31 Dec 2016, a leap second was added, and the time 23:59:60 existed.”

  1. weissbrot

    It’s fun for tech companies. The clever ones smear the second over a full day so their logs still make sense…

  2. blahcubed

    Currently, the earth is actually rotating faster than usual, meaning that we might need a **negative** leap second.

    “[There has never been a negative leap second, and if there is one, everyone who deals with NTP or kernel timekeeping code expects that it will be an appalling shitshow.](https://fanf.dreamwidth.org/133823.html)”

  3. topcat5

    Wasn’t ever a problem until they invented the Atomic Clock.

  4. tokynambu

    There are two factors at work. One, as you say, is weather and geological events. The other is that the SI second is fractionally too small in 2020, because the earth is slowing down (days for the dinosaurs were a lot shorter). Leap seconds will become quadratically more common, because they reflect the difference between a few years’ worth of SI seconds and a few years’ motion of the earth. Geological events can make the intervals between leap seconds longer (if they slightly speed up the rotation) or shorter (if they slightly slow down the rotation) but the long-term trend is going to be increasingly frequent leap seconds.

    There have been various proposals to abolish leap seconds, most recently from the ITU.

    [https://www.itu.int/net/pressoffice/press\_releases/2015/53.aspx](https://www.itu.int/net/pressoffice/press_releases/2015/53.aspx)

    The argument runs that most of us do not live in a location where mean noon is as 1200, and local solar mean time can vary from legal time by multiple hours without people objecting. Timezones change regularly, both because of daylight saving and for other legal reasons (Portugal moving between WET and UTC, UK likely to move to permanent UTC+1 or UTC+2) and therefore the likely effect of abolishing leap seconds would be to need a major realignment of timezones every few thousand years. It’s a scientific piece of kicking the can down the road.

  5. joecheph

    Morty: “That just sounds like midnight with extra steps.”

  6. OlFlirtyCraster

    Why pick the one second during the year that we count down to as a civilization? 5-4-3-2-1-1 happy new year!

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