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.

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.

Universal Time (UT)

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 length	Date
Yesterday	24 hours -0.87 ms	Wed, Jul 17, 2019
Today	24 hours -0.75 ms	Thu, Jul 18, 2019
Tomorrow	24 hours -0.63 ms	Fri, Jul 19, 2019
Shortest 2019	24 hours -0.95 ms	Mon, Jul 15, 2019
Longest 2019	24 hours +2.74 ms	Wed, Oct 16, 2019
Last Year Average	24 hours +0.69 ms	Year 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
Year	Average day	Total yearly excess	Leap second added
2019	+0.59 ms	215.47 ms	-
2018	+0.69 ms	252.46 ms	-
2017	+1.03 ms	375.01 ms	-
2016	+1.34 ms	490.76 ms	Dec 31
2015	+1.25 ms	458.03 ms	Jun 30
2014	+0.99 ms	362.96 ms	-
2013	+1.02 ms	373.99 ms	-
2012	+0.83 ms	304.11 ms	Jun 30
2011	+0.76 ms	277.94 ms	-
2010	+0.70 ms	254.74 ms	-
2009	+0.80 ms	293.37 ms	-
2008	+0.87 ms	319.49 ms	Dec 31
2007	+0.85 ms	310.81 ms	-
2006	+0.82 ms	300.88 ms	-
2005	+0.43 ms	157.76 ms	Dec 31
2004	+0.31 ms	114.01 ms	-
2003	+0.27 ms	100.16 ms	-
2002	+0.48 ms	173.79 ms	-
2001	+0.57 ms	208.94 ms	-
2000	+0.72 ms	262.42 ms	-
1999	+0.99 ms	361.19 ms	-
1998	+1.37 ms	501.72 ms	Dec 31
1997	+1.84 ms	671.08 ms	Jun 30
1996	+1.82 ms	666.37 ms	-
1995	+2.31 ms	843.66 ms	Dec 31
1994	+2.19 ms	800.86 ms	Jun 30
1993	+2.36 ms	862.66 ms	Jun 30
1992	+2.22 ms	812.25 ms	Jun 30
1991	+2.04 ms	743.88 ms	-
1990	+1.95 ms	710.04 ms	Dec 31
1989	+1.52 ms	555.00 ms	Dec 31
1988	+1.31 ms	480.30 ms	-
1987	+1.36 ms	497.35 ms	Dec 31
1986	+1.24 ms	451.06 ms	-
1985	+1.45 ms	528.83 ms	Jun 30
1984	+1.51 ms	554.42 ms	-
1983	+2.28 ms	832.08 ms	Jun 30
1982	+2.16 ms	789.64 ms	Jun 30
1981	+2.15 ms	786.03 ms	Jun 30
1980	+2.30 ms	842.04 ms	-
1979	+2.61 ms	953.02 ms	Dec 31
1978	+2.88 ms	1051.83 ms	Dec 31
1977	+2.77 ms	1012.60 ms	Dec 31
1976	+2.91 ms	1064.67 ms	Dec 31
1975	+2.69 ms	980.87 ms	Dec 31
1974	+2.72 ms	991.99 ms	Dec 31
1973	+3.04 ms	1106.21 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.