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.
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 0.7460 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|
|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.
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)
Thursday, July 18, 2019
UTC is the common time standard across the worldMore about UTC
- UTC – Coordinated Universal Time
- International Atomic Time (TAI)
- Universal Time (UT)
- Local Mean Time (LMT)
- Solar Noon
- A Day Is Not Exactly 24 Hours
- AM and PM: What Do They Mean?
- Atomic Clocks