On Friday, the international organization that decides how to keep the world's time, voted to stop something called "leap seconds."
Why these leap seconds were created, and why they won't be any more, is an interesting story.
SI second and the atomic stream of time
The science of measurement, metrology, defines the units used to measure length, mass, time, and so on. In particular, we use the SI second as a duration of time. (SI stands for the French term for International System).
We count time as a stream of SI seconds. The stream is:
Uniform, i.e, one second today is exactly as long as one second yesterday;
Continuous, i.e., one second always follows another without breaks or overlaps; and
Aligned, i.e., the seconds are counted starting from a well-known reference point or epoch.
The SI second is defined in terms of the frequency of radiation emitted by certain type of atom under certain conditions. The International Bureau of Weights and Measures (BIPM) averages the time kept by hundreds of atomic clocks all over the world to define an SI second.
The BIPM tracks and broadcasts how many SI seconds have passed since the beginning of 1958. This is called International Atomic Time, or as we might call it, the "atomic stream."
In this stream, you can show the time at any given instant either as a total number of seconds since the epoch, which will be a number in the billions, or as a calendar-clock time stamp like 2022-11-20 20:18:31. They are simply two different ways of writing the same thing, perfectly interchangeable.
This standard works very well:
A GPS receiver can use the time stamps and locations broadcast from four or more satellites to figure out where it is located.
A welding robot can coordinate its arm motions with the feed rate of the wire to produce consistent welds.
How clocks get their time: UTC
To catch a train, drop a child off to school, or attend a meeting, we use "civil time," shown by clocks and calendars. Civil time is based on the earth's rotation.
An organization of astronomers and geophysicists, IERS, specializes in measuring the rotation of the earth. Based on their measurements, the BIPM periodically broadcasts a standard universal coordinated time, UTC.
UTC defines the time at the prime meridian that passes through Greenwich, England. Civil authorities everywhere decide on a convenient offset from UTC for the timezone they want.
So, that's how clocks get their time.
Leap seconds
Unfortunately, the earth's rotation is not consistent. The average day in one year can be a few milliseconds (thousandths of a second) larger or smaller than one in another year. We know this because the IERS records its rotation measurements using SI seconds.