A Day Is Not Exactly 24 Hours
Exact Day Length* — Fri, Oct 18, 2019
Today's prediction: 24 hours, 0 minutes, 0.0001270 seconds (0.1270 milliseconds)
Yesterday: 24 hours, 0 minutes, 0.0002711 seconds (0.2711 milliseconds)
At the start of today, UT1 was 0.1531115 seconds behind UTC.
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 0.1270 ms (milliseconds) or 0.0001270 seconds longer than 24 hours. This is the time it takes Earth to rotate 5.91 cm (2.33 in), as measured at the equator.
This means that today lasts:
- 24.0000000353 hours or
- 24 hours and 0.13 ms
On average, a mean solar day in the last 365 days was 0.51 ms over 24 hours, so today's day length is below average. Over this period, 264 days have been longer than today, while 102 have been shorter than today.
If every day was as long as today, a leap second would have to be added every 7874.02 days.
|Today's Day Length* in Context|
|Yesterday||24 hours +0.27 ms||Thu, Oct 17, 2019|
|Today||24 hours +0.13 ms||Fri, Oct 18, 2019|
|Tomorrow||24 hours +0.05 ms||Sat, Oct 19, 2019|
|Shortest 2019||24 hours -0.95 ms||Tue, Jul 16, 2019|
|Longest 2019||24 hours +1.67 ms||Fri, Mar 22, 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 Thu, Oct 10, 2019 (day was 0.18 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.
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).
Coordinated Universal Time (UTC)
Saturday, October 19, 2019
UTC is the common time standard across the worldMore about UTC