en.wikipedia.org/wiki/Day

A day is approximately the period of time during which the Earth completes one rotation around .... This makes the SI-based day last exactly 794 243 384 928 000 of those periods. ... Therefore, although typically 86 400 SI seconds in duration, a civil day can be either 86 401 or 86 399 SI seconds long on such a day.

www.timeanddate.com/time/earth-rotation.html

Modern timekeeping defines a day as the sum of 24 hours – but it's not that simple. Earth's rotation slows down over time. See the exact day length for today,  ...

blog.keithw.org/2013/02/q-if-one-day-is-not-exactly-24-hours.html

You're right that a "sidereal" day is about 23 hours, 56 minutes, 4 seconds. But this is not a day in the everyday sense. A sidereal day is how long it takes the ...

medium.com/the-philipendium/a-day-is-not-24-hours-c36ee96078c6

Dec 6, 2016 ... Our system of telling time is based on the premise that every day is exactly 24 hours long — quite precisely, with no exceptions. This concept is ...

www.universetoday.com/123218/how-long-is-a-day-on-earth-2

Nov 5, 2015 ... I'm going to ask you how long a day is on Earth, and you're going to get the ... Or more specifically, how long does it take for the Earth to turn once on its axis? ..... What exactly was NASA measuring in the paper F.A.L.A. shared ...

www.quora.com/If-one-day-is-not-exactly-24-hours-and-is-in-fact-23-hours-56-minutes-shouldnt-the-error-add-up-and-shouldnt-we-see-12-midnight-becoming-12-noon-at-some-point-in-time

The Earth rotates about its axis exactly once, or 360 degrees, in about 23 hours and 56 minutes. ... How long is a day, 12 hours or 24 hours?

physics.stackexchange.com/questions/469869/how-can-a-day-be-exactly-24-hours-long

You can't calculate the length of a mean solar day just by taking the mean of the shortest & longest apparent solar days. That would work if the ...

www.jpl.nasa.gov/news/news.php?feature=15

Jan 22, 2002 ... A classic movie about World War II's D-Day was called "The Longest Day. ... Knowing exactly when and how much Earth's rotation varies may lead to ... very long baseline interferometry; and the global positioning system.

www.days-until-christmas.co.uk/exact-time-til-xmas.htm

Our Exact Time Until Christmas Day clock will show you exactly how long you have left until Xmas Day. If you want to display the exact time till Christmas on your ...