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A Day Is Not Exactly 24 Hours

Exact Day Length* — Sat, 17 Aug 2019

Today's prediction: 23 hours, 59 minutes, 59.9996554 seconds (-0.3446 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9996077 seconds (-0.3923 milliseconds)

At the start of today, UT1 was 0.1559673 seconds behind UTC.

Look up a day length in the past

Star trails over an observatory.

The Earth's rotation slows down over time.

Startrails lining the sky over an observatory. The Earth's rotation slows down over time.


Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. The Earth's rotation slows down over time. So in terms of solar time, most days are a little longer than 24 hours.

How Long Is Today?

Today is predicted to be 0.3446 ms (milliseconds) or 0.0003446 seconds shorter than 24 hours. This is the time it takes Earth to rotate 16.03 cm (6.31 in), as measured at the equator.

This means that today lasts:

  • 23.9999999043 hours or
  • 24 hours minus 0.34 ms

On average, a mean solar day in the last 365 days was 0.60 ms over 24 hours, so today's day length is below average. Over this period, 337 days have been longer than today, while 29 have been shorter than today.

If every day was as long as today, a negative leap second would have to be added every 2901.92 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours -0.39 msFri, 16 Aug 2019
Today24 hours -0.34 msSat, 17 Aug 2019
Tomorrow24 hours -0.28 msSun, 18 Aug 2019
Shortest 201924 hours -0.96 msTue, 16 Jul 2019
Longest 201924 hours +1.67 msFri, 22 Mar 2019
Last Year Average24 hours +0.69 msYear 2018
* Yesterday's, today's and future day lengths are predictions.

Average Day Lengths & Leap Seconds

In rare cases, a day can also be shorter than 24 hours. The last time this happened was on Thu, 15 Aug 2019 (day was 0.45 ms short). However, the average day exceeds 24 hours. See the table below for yearly average day lengths.

To make up for the additional duration, leap seconds are added to our clocks from time to time.

Average Solar Day Length
YearAverage dayTotal yearly excessLeap second added
2019+0.55 ms200.77 ms-
2018+0.69 ms252.47 ms-
2017+1.03 ms375.01 ms-
2016+1.34 ms490.76 ms31 Dec
2015+1.25 ms458.03 ms30 Jun
2014+0.99 ms362.96 ms-
2013+1.02 ms373.99 ms-
2012+0.83 ms304.11 ms30 Jun
2011+0.76 ms277.94 ms-
2010+0.70 ms254.74 ms-
2009+0.80 ms293.37 ms-
2008+0.87 ms319.49 ms31 Dec
2007+0.85 ms310.81 ms-
2006+0.82 ms300.88 ms-
2005+0.43 ms157.76 ms31 Dec
2004+0.31 ms114.01 ms-
2003+0.27 ms100.16 ms-
2002+0.48 ms173.79 ms-
2001+0.57 ms208.94 ms-
2000+0.72 ms262.42 ms-
1999+0.99 ms361.19 ms-
1998+1.37 ms501.72 ms31 Dec
1997+1.84 ms671.08 ms30 Jun
1996+1.82 ms666.37 ms-
1995+2.31 ms843.66 ms31 Dec
1994+2.19 ms800.86 ms30 Jun
1993+2.36 ms862.66 ms30 Jun
1992+2.22 ms812.25 ms30 Jun
1991+2.04 ms743.88 ms-
1990+1.95 ms710.04 ms31 Dec
1989+1.52 ms555.00 ms31 Dec
1988+1.31 ms480.30 ms-
1987+1.36 ms497.35 ms31 Dec
1986+1.24 ms451.06 ms-
1985+1.45 ms528.83 ms30 Jun
1984+1.51 ms554.42 ms-
1983+2.28 ms832.08 ms30 Jun
1982+2.16 ms789.64 ms30 Jun
1981+2.15 ms786.03 ms30 Jun
1980+2.30 ms842.04 ms-
1979+2.61 ms953.02 ms31 Dec
1978+2.88 ms1051.83 ms31 Dec
1977+2.77 ms1012.60 ms31 Dec
1976+2.91 ms1064.67 ms31 Dec
1975+2.69 ms980.87 ms31 Dec
1974+2.72 ms991.99 ms31 Dec
1973+3.04 ms1106.21 ms31 Dec
* Current year's average day length and total yearly excess are predicted.

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Why Are the Days Getting Longer?

The speed of the Earth's rotation decreases over time, but it also varies from day to day. One of the main factors are the celestial bodies surrounding us. For example, the Moon's gravitational pull causes tides and changes the Earth's shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly, which makes for daily variations in the speed our planet rotates around its own axis.

Find Day Length for any Date


(*) Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).

Topics: Earth, Timekeeping, Astronomy

Coordinated Universal Time (UTC)


Saturday, 17 August 2019

UTC is the common time standard across the world

More about UTC

Keeping Time

  1. UTC – Coordinated Universal Time
  2. International Atomic Time (TAI)
  3. Universal Time (UT)
  4. Local Mean Time (LMT)
  5. Solar Noon
  6. A Day Is Not Exactly 24 Hours
  7. AM and PM: What Do They Mean?
  8. Atomic Clocks

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