Because the exact length of a year is a little less than 365.25 days, how long will it be until the actual time and the calendar are at least 24 hours off?

The lengths of the year, month, and day are not strictly constant. Solar tidal friction is slowly increasing the length of the year by moving Earth away from the Sun; lunar tidal friction is lengthening the month and the day by moving the Moon away from Earth and causing the earth to rotate more slowly. The lengthening of the earth's year is negligibly slow, about one billionth of a year every billion years, but the lengthening of Earth's day is much faster: about two seconds every 10,000 years. At the beginning of the Cambrian Period, for example, approximately 600 million years ago, there were over 420 days in each year, each only 21 hours long.

Most people are probably unaware of this but the length of a solar day, which is the natural day measured by the rising and setting of the Sun isn’t always 24 hours. It varies slightly throughout the course of the year and the days in mid September are the shortest solar days in the year. This post discusses this curiosity, which is not widely known … Although a day for practical timekeeping purposes is always 24 hours, the actual length of a solar day, which is the time difference between two successive occasions when the Sun is at its highest in the sky, varies throughout the year. As shown in the graph below, it is at its longest, 24 hours 30 seconds, around Christmas Day and is at its shortest, 23 hours 59 minutes 38 seconds, in mid-September.