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A Day Is Not Exactly 24 Hours

Exact Day Length* — Thu, Jul 18, 2019

Today's prediction: 23 hours, 59 minutes, 59.9992540 seconds (-0.7460 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9991337 seconds (-0.8663 milliseconds)

At the start of today, UT1 was 0.1669981 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.

©iStockphoto.com/nukleerkedi

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 0.7460 ms (milliseconds) or 0.0007460 seconds shorter than 24 hours. This is the time it takes Earth to rotate 34.70 cm (13.66 in), as measured at the equator.

This means that today lasts:

  • 23.9999997928 hours or
  • 24 hours minus 0.75 ms

On average, a mean solar day in the last 365 days was 0.65 ms over 24 hours, so today's day length is below average. Over this period, 362 days have been longer than today, while 4 have been shorter than today.

If every day was as long as today, a negative leap second would have to be added every 1340.48 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours -0.87 msWed, Jul 17, 2019
Today24 hours -0.75 msThu, Jul 18, 2019
Tomorrow24 hours -0.63 msFri, Jul 19, 2019
Shortest 201924 hours -0.95 msMon, Jul 15, 2019
Longest 201924 hours +2.74 msWed, Oct 16, 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 Tue, Jul 16, 2019 (day was 0.94 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+0.59 ms215.47 ms-
2018+0.69 ms252.46 ms-
2017+1.03 ms375.01 ms-
2016+1.34 ms490.76 msDec 31
2015+1.25 ms458.03 msJun 30
2014+0.99 ms362.96 ms-
2013+1.02 ms373.99 ms-
2012+0.83 ms304.11 msJun 30
2011+0.76 ms277.94 ms-
2010+0.70 ms254.74 ms-
2009+0.80 ms293.37 ms-
2008+0.87 ms319.49 msDec 31
2007+0.85 ms310.81 ms-
2006+0.82 ms300.88 ms-
2005+0.43 ms157.76 msDec 31
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 msDec 31
1997+1.84 ms671.08 msJun 30
1996+1.82 ms666.37 ms-
1995+2.31 ms843.66 msDec 31
1994+2.19 ms800.86 msJun 30
1993+2.36 ms862.66 msJun 30
1992+2.22 ms812.25 msJun 30
1991+2.04 ms743.88 ms-
1990+1.95 ms710.04 msDec 31
1989+1.52 ms555.00 msDec 31
1988+1.31 ms480.30 ms-
1987+1.36 ms497.35 msDec 31
1986+1.24 ms451.06 ms-
1985+1.45 ms528.83 msJun 30
1984+1.51 ms554.42 ms-
1983+2.28 ms832.08 msJun 30
1982+2.16 ms789.64 msJun 30
1981+2.15 ms786.03 msJun 30
1980+2.30 ms842.04 ms-
1979+2.61 ms953.02 msDec 31
1978+2.88 ms1051.83 msDec 31
1977+2.77 ms1012.60 msDec 31
1976+2.91 ms1064.67 msDec 31
1975+2.69 ms980.87 msDec 31
1974+2.72 ms991.99 msDec 31
1973+3.04 ms1106.21 msDec 31
* 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

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(*) 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)

11:39:37

Thursday, July 18, 2019

UTC is the common time standard across the world

More about UTC
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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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