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A Day Is Not Exactly 24 Hours

Exact Day Length* — Sun, Jan 17, 2021

Today's prediction: 23 hours, 59 minutes, 59.9998124 seconds (-0.1876 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9997231 seconds (-0.2769 milliseconds)

At the start of today, UT1 was 0.1717624 seconds behind UTC.

Star trails over an observatory.

The Earth's rotation slows down over time.

©iStockphoto.com/nukleerkedi

Earth's Rotation Defines Length of Day

Modern timekeeping defines a day as the sum of 24 hours—but that is not entirely correct. The Earth's rotation is not constant, so in terms of solar time, most days are a little longer or shorter than that.

In past decades, the Earth's average rotational speed has consistently decreased, so most days were a little longer than 24 hours. Recently, however, our planet has been spinning faster again, making for slightly shorter days.

How Long Is Today?

Today is predicted to be 0.1876 ms (milliseconds) or 0.0001876 seconds shorter than 24 hours. This is the time it takes Earth to rotate 8.73 cm (3.44 in), as measured at the equator.

This means that today lasts:

  • 23.9999999479 hours or
  • 24 hours minus 0.19 ms

On average, a mean solar day in the last 365 days was -0.03 ms under 24 hours, so today's day length is below average. Over this period, 215 days have been longer than today, while 151 have been shorter than today.

If every day was as long as today, a negative leap second would have to be added every 5330.49 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours -0.28 msSat, Jan 16, 2021
Today24 hours -0.19 msSun, Jan 17, 2021
Tomorrow24 hours -0.16 msMon, Jan 18, 2021
Shortest 202124 hours -1.72 msThu, Jul 22, 2021
Longest 202124 hours +1.29 msWed, Nov 3, 2021
Last Year Average24 hours -0.00 msYear 2020
* 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 Fri, Jan 15, 2021 (day was 0.42 ms short). However, the average day exceeds 24 hours most years. 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 excessShortest dayLongest dayLeap second added
2021-0.12 ms-44.89 msJul 22 -1.72 msNov 3 +1.29 ms-
2020-0.00 ms-1.28 msJul 19 -1.46 msApr 8 +1.62 ms-
2019+0.39 ms+141.25 msJul 16 -0.95 msMar 22 +1.68 ms-
2018+0.69 ms+252.47 msJun 30 -0.64 msFeb 4 +1.69 ms-
2017+1.03 ms+375.01 msAug 4 +0.06 msApr 25 +2.20 ms-
2016+1.34 ms+490.76 msJul 18 -0.03 msMar 10 +2.49 msDec 31
2015+1.25 ms+458.03 msJun 17 +0.19 msOct 26 +2.31 msJun 30
2014+0.99 ms+362.96 msJul 24 +0.02 msApr 26 +2.02 ms-
2013+1.02 ms+373.99 msJul 6 -0.35 msMar 28 +1.97 ms-
2012+0.83 ms+304.11 msJul 16 -0.35 msApr 5 +1.87 msJun 30
2011+0.76 ms+277.94 msJul 27 -0.34 msMay 14 +1.85 ms-
2010+0.70 ms+254.74 msJul 23 -0.76 msMar 1 +2.09 ms-
2009+0.80 ms+293.37 msJul 6 -0.43 msApr 22 +1.81 ms-
2008+0.87 ms+319.49 msJul 16 -0.41 msApr 5 +1.91 msDec 31
2007+0.85 ms+310.81 msJul 27 -0.63 msApr 16 +2.31 ms-
2006+0.82 ms+300.88 msJun 12 -0.40 msOct 7 +2.26 ms-
2005+0.43 ms+157.76 msJul 5 -1.05 msFeb 27 +1.73 msDec 31
2004+0.31 ms+114.01 msJul 15 -1.05 msApr 5 +1.56 ms-
2003+0.27 ms+100.16 msJul 13 -0.96 msMar 19 +1.55 ms-
2002+0.48 ms+173.79 msAug 6 -0.74 msMar 2 +1.66 ms-
2001+0.57 ms+208.94 msAug 2 -0.71 msMar 11 +1.64 ms-
2000+0.72 ms+262.42 msAug 11 -0.25 msOct 26 +1.58 ms-
1999+0.99 ms+361.19 msJun 30 -0.13 msApr 15 +1.93 ms-
1998+1.37 ms+501.72 msJul 9 +0.01 msMar 1 +2.66 msDec 31
1997+1.84 ms+671.08 msJul 4 +0.52 msApr 6 +2.98 msJun 30
1996+1.82 ms+666.37 msAug 10 +0.67 msMay 12 +2.68 ms-
1995+2.31 ms+843.66 msJul 25 +0.81 msMar 17 +3.29 msDec 31
1994+2.19 ms+800.86 msJul 6 +0.86 msFeb 27 +3.36 msJun 30
1993+2.36 ms+862.66 msJul 17 +1.25 msMay 2 +3.49 msJun 30
1992+2.22 ms+812.25 msJul 12 +0.84 msMar 18 +3.59 msJun 30
1991+2.04 ms+743.88 msJun 27 +0.79 msMar 1 +3.00 ms-
1990+1.95 ms+710.04 msJul 20 +0.63 msMar 26 +3.28 msDec 31
1989+1.52 ms+555.00 msJul 2 +0.25 msNov 10 +2.82 msDec 31
1988+1.31 ms+480.30 msJul 12 -0.09 msFeb 20 +2.76 ms-
1987+1.36 ms+497.35 msJul 23 -0.06 msMar 1 +2.67 msDec 31
1986+1.24 ms+451.06 msAug 2 -0.04 msApr 23 +2.30 ms-
1985+1.45 ms+528.83 msJul 16 +0.11 msMar 9 +2.64 msJun 30
1984+1.51 ms+554.42 msJul 12 +0.16 msMar 18 +2.77 ms-
1983+2.28 ms+832.08 msJul 23 +1.01 msFeb 1 +3.57 msJun 30
1982+2.16 ms+789.64 msAug 2 +0.84 msApr 23 +3.14 msJun 30
1981+2.15 ms+786.03 msJul 16 +0.82 msMar 8 +3.42 msJun 30
1980+2.30 ms+842.04 msAug 8 +1.34 msOct 23 +3.24 ms-
1979+2.61 ms+953.02 msJul 23 +1.46 msMar 27 +3.65 msDec 31
1978+2.88 ms+1051.83 msJul 31 +1.49 msMar 9 +3.83 msDec 31
1977+2.77 ms+1012.60 msJul 14 +1.46 msApr 4 +3.72 msDec 31
1976+2.91 ms+1064.67 msJun 26 +1.87 msOct 21 +3.90 msDec 31
1975+2.69 ms+980.87 msJul 20 +1.54 msNov 1 +3.72 msDec 31
1974+2.72 ms+991.99 msJul 30 +1.57 msApr 5 +3.79 msDec 31
1973+3.04 ms+1106.21 msJan 2 +0.00 msApr 2 +4.03 msDec 31
* Current year's average day length and total yearly excess are predicted.

How Is True Day Length Measured?

Astronomers and timekeepers express mean solar time as Universal Time (UT1), a time standard based on the average speed of the Earth's rotation. UT1 is then compared to International Atomic Time (TAI), a super-precise time scale calculated by a network of atomic clocks.

The actual length of a day is expressed as the deviation of UT1 from TAI over 24 hours.

Why Isn't Earth's Rotation Constant?

The speed of the Earth's rotation 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 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