Home   Time Zones   A Day's Length

A Day Is Not Exactly 24 Hours

Exact Day Length* — Sat, Oct 20, 2018

Today's prediction: 24 hours, 0 minutes, 0.0007917 seconds (0.7917 milliseconds)

Yesterday's prediction: 24 hours, 0 minutes, 0.0006468 seconds (0.6468 milliseconds)

At the start of today, UT1 was 0.0319763 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 0.7917 ms (milliseconds) or 0.0007917 seconds longer than 24 hours. This is the time it takes Earth to rotate 36.82 cm (14.50 in), as measured at the equator.

This means that today lasts:

  • 24.0000002199 hours or
  • 24 hours and 0.79 ms

On average, a mean solar day in the last 365 days was 0.72 ms over 24 hours, so today's day length is above average. Over this period, 182 days have been longer than today, while 184 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 1263.10 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours +0.65 msFri, Oct 19, 2018
Today24 hours +0.79 msSat, Oct 20, 2018
Tomorrow24 hours +0.95 msSun, Oct 21, 2018
Shortest 201824 hours -0.64 msSat, Jun 30, 2018
Longest 201824 hours +1.69 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 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
YearAverage dayTotal yearly excessLeap second added
2018+0.69 ms251.72 ms-
2017+1.03 ms375.01 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)

13:24:27

Saturday, October 20, 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

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

What Is the Antumbra?

The antumbra is the lighter part of a shadow that forms at a certain distance from the object casting the shadow. It is involved in annular solar eclipses and planet transits. more