The AlgorithmicRule for Leap Years and Leap Seconds
To keep our Gregorian calendar in sync with mean solar time (UT1 time scale), we need to add a second once in a while in addition to adding a day every four years. However, there is more complexity that we need to consider.
As a Computer Systems Programmer, I once had to write an algorithm to determine the day of the week (Monday, Tuesday, etc.) for any specific calendar day. That required a thorough understanding of how we calculate the days of our calendar. So, I can explain it to you.
Extremely Accurate Time Measurements
We live in a time when we have the resources to do extremely accurate measurements. We have the technology to measure the rotation of the Earth so precisely that we can detect how it is slowing down. We use atomic clocks to keep an accurate account of time.
There are National Standards Agencies in many countries that maintain a network of atomic clocks. They are kept synchronized with extreme accuracy.
In addition, we have the master atomic clock at the U.S. Naval Observatory in Washington, providing the time standard for the U.S. Department of Defense.
The NISTF1, a cesium fountain atomic clock developed at the NIST laboratories in 2013 at Boulder, Colorado, is more accurate than previous atomic clocks.^{1}
Managing The Extra Fractions of Days
If it takes exactly 365 days for the Earth to revolve around the Sun, then we would have a perfect calendar, and we would not need to make corrections.
If a year had 365 and a quarter days precisely, then adding a day every four years would work wonderfully. Unfortunately, our Earth goes around the Sun in 365.2426 days, so adding a day every four years is adding too much.
We add an extra day, February 29th, every four years. However, we need to skip that addition once in a while.
I'll get a little deeper into the math to explain the details. If that additional fraction over 365 days were exactly .25 (an extra quarter of a day), then every four years adds up to a full day. If that were accurate, we would just add that extra day at the end of February every four years, but we don't. We need to skip some Leap Years.
We Need to Skip a Leap Year Every 100 Years
That fraction I mentioned before, 0.2426, is a little less than a quarter of a day. Therefore, every 100 years, we need to skip the addition of a day in February—otherwise, we would be adding too much.
Skipping a leap year every 100 years only works if the extra time were exactly 0.25. However, we are still off by almost .01 from a quarter day. That .01 adds up to 1 in 100 years. Therefore we need to skip a leap year every 100 years. If we didn't skip it, we'd be adding too many days to the calendar.
But wait! That still isn't perfect! We'll still get out of sync with solar time if we don't take it a step further.
We Need to Add an Extra Day Every 400 Years
As you can see, we still have that extra .0026 that we are off when skipping a leap year every 100 years. If you add that up, with some rounding error, that .0026 is a little over one day every 400 years (.0026 x 400 = 1.04).
That means that skipping a leap year every 100 years needs adjustment as well. We need to add a day back in. We need to keep that leap year every 400 years in order to get that one extra day added back.
The easiest way to add that missing day back in is "not to skip" a leap year when the year is a multiple of 400. In other words, we keep February 29th on the calendar every 400 years even though it is a multiple of 100.
Explaining It in One Sentence
To say it all in one sentence: We add a day every four years, but not every 100 years, unless it’s a 4th century year, at which point we do add that extra day anyway.
Table of Leap Years and the Reasons For It
Year
 Leap Year if Multiple of 4
 But Skip if Multiple of 100
 Unless it's a Multiple of 400
 Leap Year?


1600
 Yes
 
 Yes
 Yes

1700
 Yes
 Yes
 No
 No

1800
 Yes
 Yes
 No
 No

1900
 Yes
 Yes
 No
 No

2000
 Yes
 
 Yes
 Yes

2004
 Yes
 No
 
 Yes

2008
 Yes
 No
 
 Yes

2012
 Yes
 No
 
 Yes

2016
 Yes
 No
 
 Yes

2100
 Yes
 Yes
 No
 No

Leap Seconds are Needed Too
The algorithm for leap years still does not provide perfect accuracy. Adding a few seconds is also required. Climate and geological events can cause the Earth’s revolution around the Sun to fluctuate.
In addition to that, the Earth's rotation around its own axis is not consistent. It tends to slow down and speed up ever so little.
A 9.0 magnitude Earthquake in Japan in 2011 had shifted the Earth's axis by an amount between 10 cm (4 inches) and 25 cm (10 inches). These fluctuations can change the length of a day by a tiny amount, and we need to adjust our calendar accordingly.^{2}
To improve the accuracy of our time clocks, we need to add a second or two every year. It’s called a leap second.^{3 }
Scheduling the addition of an extra second to a year is done to make these adjustments.
It's usually added, when needed, as an additional second at midnight, Coordinated Universal Time (UTC), on June 30th or December 31st.
Prior Leap Second Time Adjustments
The International Earth Rotation and Reference Systems Service is the agency that decides when to make leap second time adjustments. They apply a leap second whenever necessary to keep our clock from being more than 0.9 of a second off.
Here is a table of dates when a second was added. Each addition was at midnight (UTC):
 December 31, 2008
 December 31, 2005
 June 30, 2012
 June 30, 2015
 December 31, 2016
 June 30, 2018
Final Thoughts
Physical events, such as earthquakes, can nudge the Earth just enough to require adding another leap second so that our clocks remain in sync with the way we represent time.
It's an ongoing struggle to keep our time measurements as accurate as possible. With the present technology, we have the means to do that.
References
 Physical Measurement Laboratory. (October 19, 2018).“NISTF1 Cesium Fountain Atomic Clock.” NIST Time and Frequency Division
 "2011 Tōhoku earthquake and tsunami." Wikipedia
 "Leap second." Wikipedia
Questions & Answers
© 2012 Glenn Stok
Comments
The calculation .0026 x 400 = 1.04 shows that we fall behind by 1.04 days every 400 years and explains why we need to add an extra leap day every 400 years. But what about the .04 days that we still fall behind every 400 years. .04 x 24 hours = .96 or about 1 hour every 400 years. You indicated a concern for compensating for lost seconds with leap seconds. Is any compensation made for that lost .96 hour every 400 years?
My pleasure Glenn. I always wondered about the leap year and never heard of leap seconds.
Glen, this was real interesting to know we had a extra leap second last month and next year is a leap year. This was wellwritten and intriguing to know the history behind the leap year. Voted up!
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