A Day Is Not Exactly 24 Hours

Exact Day Length* — Thu, Oct 1, 2020

Today's prediction: 23 hours, 59 minutes, 59.9998494 seconds (-0.1506 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9997990 seconds (-0.2010 milliseconds)

At the start of today, UT1 was 0.1735219 seconds behind UTC.

Star trails 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.

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

How Long Is Today?

Today is predicted to be 0.1506 ms (milliseconds) or 0.0001506 seconds shorter than 24 hours. This is the time it takes Earth to rotate 7.00 cm (2.76 in), as measured at the equator.

This means that today lasts:

23.9999999582 hours or
24 hours minus 0.15 ms

On average, a mean solar day in the last 365 days was 0.05 ms over 24 hours, so today's day length is below average. Over this period, 244 days have been longer than today, while 122 have been shorter than today.

If every day was as long as today, a negative leap second would have to be added every 6640.11 days.

Today's Day Length* in Context
	Day length	Date
Yesterday	24 hours -0.20 ms	Wed, Sep 30, 2020
Today	24 hours -0.15 ms	Thu, Oct 1, 2020
Tomorrow	24 hours -0.14 ms	Fri, Oct 2, 2020
Shortest 2020	24 hours -1.46 ms	Sun, Jul 19, 2020
Longest 2020	24 hours +6.12 ms	Wed, Dec 30, 2020
Last Year Average	24 hours +0.39 ms	Year 2019
* 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, Sep 29, 2020 (day was 0.37 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
Year	Average day	Total yearly excess	Shortest day	Longest day	Leap second added
2020	+0.01 ms	4.42 ms	Jul 19 -1.46 ms	Dec 30 +6.12 ms	-
2019	+0.39 ms	141.25 ms	Jul 16 -0.95 ms	Mar 22 +1.68 ms	-
2018	+0.69 ms	252.47 ms	Jun 30 -0.64 ms	Feb 4 +1.69 ms	-
2017	+1.03 ms	375.01 ms	Aug 4 +0.06 ms	Apr 25 +2.20 ms	-
2016	+1.34 ms	490.76 ms	Jul 18 -0.03 ms	Mar 10 +2.49 ms	Dec 31
2015	+1.25 ms	458.03 ms	Jun 17 +0.19 ms	Oct 26 +2.31 ms	Jun 30
2014	+0.99 ms	362.96 ms	Jul 24 +0.02 ms	Apr 26 +2.02 ms	-
2013	+1.02 ms	373.99 ms	Jul 6 -0.35 ms	Mar 28 +1.97 ms	-
2012	+0.83 ms	304.11 ms	Jul 16 -0.35 ms	Apr 5 +1.87 ms	Jun 30
2011	+0.76 ms	277.94 ms	Jul 27 -0.34 ms	May 14 +1.85 ms	-
2010	+0.70 ms	254.74 ms	Jul 23 -0.76 ms	Mar 1 +2.09 ms	-
2009	+0.80 ms	293.37 ms	Jul 6 -0.43 ms	Apr 22 +1.81 ms	-
2008	+0.87 ms	319.49 ms	Jul 16 -0.41 ms	Apr 5 +1.91 ms	Dec 31
2007	+0.85 ms	310.81 ms	Jul 27 -0.63 ms	Apr 16 +2.31 ms	-
2006	+0.82 ms	300.88 ms	Jun 12 -0.40 ms	Oct 7 +2.26 ms	-
2005	+0.43 ms	157.76 ms	Jul 5 -1.05 ms	Feb 27 +1.73 ms	Dec 31
2004	+0.31 ms	114.01 ms	Jul 15 -1.05 ms	Apr 5 +1.56 ms	-
2003	+0.27 ms	100.16 ms	Jul 13 -0.96 ms	Mar 19 +1.55 ms	-
2002	+0.48 ms	173.79 ms	Aug 6 -0.74 ms	Mar 2 +1.66 ms	-
2001	+0.57 ms	208.94 ms	Aug 2 -0.71 ms	Mar 11 +1.64 ms	-
2000	+0.72 ms	262.42 ms	Aug 11 -0.25 ms	Oct 26 +1.58 ms	-
1999	+0.99 ms	361.19 ms	Jun 30 -0.13 ms	Apr 15 +1.93 ms	-
1998	+1.37 ms	501.72 ms	Jul 9 +0.01 ms	Mar 1 +2.66 ms	Dec 31
1997	+1.84 ms	671.08 ms	Jul 4 +0.52 ms	Apr 6 +2.98 ms	Jun 30
1996	+1.82 ms	666.37 ms	Aug 10 +0.67 ms	May 12 +2.68 ms	-
1995	+2.31 ms	843.66 ms	Jul 25 +0.81 ms	Mar 17 +3.29 ms	Dec 31
1994	+2.19 ms	800.86 ms	Jul 6 +0.86 ms	Feb 27 +3.36 ms	Jun 30
1993	+2.36 ms	862.66 ms	Jul 17 +1.25 ms	May 2 +3.49 ms	Jun 30
1992	+2.22 ms	812.25 ms	Jul 12 +0.84 ms	Mar 18 +3.59 ms	Jun 30
1991	+2.04 ms	743.88 ms	Jun 27 +0.79 ms	Mar 1 +3.00 ms	-
1990	+1.95 ms	710.04 ms	Jul 20 +0.63 ms	Mar 26 +3.28 ms	Dec 31
1989	+1.52 ms	555.00 ms	Jul 2 +0.25 ms	Nov 10 +2.82 ms	Dec 31
1988	+1.31 ms	480.30 ms	Jul 12 -0.09 ms	Feb 20 +2.76 ms	-
1987	+1.36 ms	497.35 ms	Jul 23 -0.06 ms	Mar 1 +2.67 ms	Dec 31
1986	+1.24 ms	451.06 ms	Aug 2 -0.04 ms	Apr 23 +2.30 ms	-
1985	+1.45 ms	528.83 ms	Jul 16 +0.11 ms	Mar 9 +2.64 ms	Jun 30
1984	+1.51 ms	554.42 ms	Jul 12 +0.16 ms	Mar 18 +2.77 ms	-
1983	+2.28 ms	832.08 ms	Jul 23 +1.01 ms	Feb 1 +3.57 ms	Jun 30
1982	+2.16 ms	789.64 ms	Aug 2 +0.84 ms	Apr 23 +3.14 ms	Jun 30
1981	+2.15 ms	786.03 ms	Jul 16 +0.82 ms	Mar 8 +3.42 ms	Jun 30
1980	+2.30 ms	842.04 ms	Aug 8 +1.34 ms	Oct 23 +3.24 ms	-
1979	+2.61 ms	953.02 ms	Jul 23 +1.46 ms	Mar 27 +3.65 ms	Dec 31
1978	+2.88 ms	1051.83 ms	Jul 31 +1.49 ms	Mar 9 +3.83 ms	Dec 31
1977	+2.77 ms	1012.60 ms	Jul 14 +1.46 ms	Apr 4 +3.72 ms	Dec 31
1976	+2.91 ms	1064.67 ms	Jun 26 +1.87 ms	Oct 21 +3.90 ms	Dec 31
1975	+2.69 ms	980.87 ms	Jul 20 +1.54 ms	Nov 1 +3.72 ms	Dec 31
1974	+2.72 ms	991.99 ms	Jul 30 +1.57 ms	Apr 5 +3.79 ms	Dec 31
1973	+3.04 ms	1106.21 ms	Jan 2 +0.00 ms	Apr 2 +4.03 ms	Dec 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

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

Topics: Earth, Timekeeping, Astronomy