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A Day is Not Excatly 24 Hours

Exact Day Length* — Sun, Mar 26, 2017

Today's prediction: 24 hours, 0 minutes, 0.0018305 seconds (1.8305 milliseconds)

Yesterday's prediction: 24 hours, 0 minutes, 0.0016738 seconds (1.6738 milliseconds)

At the start of today, UT1 was 0.4816796 seconds ahead of 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 1.8305 ms (milliseconds) or 0.0018305 seconds longer than 24 hours. This is the time it takes Earth to rotate 85.14 cm (33.52 in), as measured at the equator.

This means that today lasts:

  • 24.0000005085 hours or
  • 24 hours and 1.83 ms

On average, a mean solar day in the last 365 days was 1.23 ms over 24 hours, so today's day length is above average. Over this period, 32 days have been longer than today, while 334 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 546.30 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours +1.67 msSat, Mar 25, 2017
Today24 hours +1.83 msSun, Mar 26, 2017
Tomorrow24 hours +1.94 msMon, Mar 27, 2017
Shortest 201724 hours -0.11 msSun, Jul 9, 2017
Longest 201724 hours +2.04 msFri, Nov 3, 2017
Last Year Average24 hours +1.34 msYear 2016
* 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 Mon, Jul 18, 2016 (day was 0.02 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
2017+1.12 ms407.81 ms-
2016+1.34 ms490.82 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 ms980.86 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 Becoming 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).

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