Home   Time Zones   A Day's Length

A Day Is Not Exactly 24 Hours

Exact Day Length* — Thu, Aug 16, 2018

Today's prediction: 24 hours, 0 minutes, 0.0004823 seconds (0.4823 milliseconds)

Yesterday's prediction: 24 hours, 0 minutes, 0.0005816 seconds (0.5816 milliseconds)

At the start of today, UT1 was 0.0651548 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 0.4823 ms (milliseconds) or 0.0004823 seconds longer than 24 hours. This is the time it takes Earth to rotate 22.43 cm (8.83 in), as measured at the equator.

This means that today lasts:

  • 24.0000001340 hours or
  • 24 hours and 0.48 ms

On average, a mean solar day in the last 365 days was 0.76 ms over 24 hours, so today's day length is below average. Over this period, 275 days have been longer than today, while 91 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 2073.40 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours +0.58 msWed, Aug 15, 2018
Today24 hours +0.48 msThu, Aug 16, 2018
Tomorrow24 hours +0.33 msFri, Aug 17, 2018
Shortest 201824 hours -2.24 msWed, Nov 14, 2018
Longest 201824 hours +1.70 msSun, Feb 4, 2018
Last Year Average24 hours +1.03 msYear 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 Mon, Jul 30, 2018 (day was 0.03 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
2018+0.72 ms264.29 ms-
2017+1.03 ms375.02 ms-
2016+1.34 ms490.76 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 ms980.87 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.

Subscribe to our newsletter for leap second alerts

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

Coordinated Universal Time (UTC)

08:10:46

Thursday, August 16, 2018

UTC is the common time standard across the world

More about UTC
Advertising

Keeping Time

  1. UTC – Coordinated Universal Time
  2. International Atomic Time (TAI)
  3. Universal Time (UT)
  4. Local Mean Time (LMT)
  5. Solar Noon
  6. A Day Is Not Exactly 24 Hours
  7. AM and PM: What Do They Mean?
  8. Atomic Clocks

You might also like

Look Up for Perseids Meteors

Look Up for Perseids Meteors

The Perseid meteor shower is still active in the night sky. Try our Interactive Meteor Shower Sky Map to see when and where to spot these shooting stars. more

A sundial casting a shadow.

Equation of Time

Why doesn't the year's earliest sunset occur on the winter solstice, even though it is the shortest day of the year in terms of daylight? more

Why 3 Shadows?

The Earth and the Moon cast 3 different shadows: umbra, penumbra, and antumbra. Why are there 3 types of shadows and how are they defined? more