A Day Is Not Exactly 24 Hours

Exact Day Length* — Sat, Dec 15, 2018

Today's prediction: 24 hours, 0 minutes, 0.0008653 seconds (0.8653 milliseconds)

Yesterday's prediction: 24 hours, 0 minutes, 0.0007239 seconds (0.7239 milliseconds)

At the start of today, UT1 was 0.0222527 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 0.8653 ms (milliseconds) or 0.0008653 seconds longer than 24 hours. This is the time it takes Earth to rotate 40.25 cm (15.84 in), as measured at the equator.

This means that today lasts:

24.0000002404 hours or
24 hours and 0.87 ms

On average, a mean solar day in the last 365 days was 0.69 ms over 24 hours, so today's day length is above average. Over this period, 151 days have been longer than today, while 215 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 1155.67 days.

Today's Day Length* in Context
	Day length	Date
Yesterday	24 hours +0.72 ms	Fri, Dec 14, 2018
Today	24 hours +0.87 ms	Sat, Dec 15, 2018
Tomorrow	24 hours +0.99 ms	Sun, Dec 16, 2018
Shortest 2018	24 hours -0.64 ms	Sat, Jun 30, 2018
Longest 2018	24 hours +1.69 ms	Sun, Feb 4, 2018
Last Year Average	24 hours +1.03 ms	Year 2017
* 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 Sat, Aug 25, 2018 (day was 0.15 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	+0.63 ms	229.02 ms	-
2018	+0.70 ms	254.01 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.