And there is a pattern! Over a half century of records now, the shortest day of each year has occurred in June, July, or August. Meanwhile, the longest day has usually been sometime between March and May, but occasionally happened in October or November. There is even a shorter pattern of oscillations overlaid on top of that longer pattern, where the length of day swings back and forth between longer and shorter every 5-8 days or so.
This graph of length of day (in milliseconds over (+) or under (-1) the average of 86,400,000 ms) shows the unusual pattern in the lengthening and shortening of our days here on Earth. The data represented, ranging from April 17 through July 14, 2025, is the latest data available from the IERS as of July 16. (Data from IERS)
According to the International Earth Rotation and Reference Systems Service (IERS), in 2025, the longest day of the year so far was Saturday, March 29, at 1.12 milliseconds longer than usual.
The shortest day of the year, so far, was Thursday, July 10, at 1.37 milliseconds shorter than usual.
That’s not the shortest day in history. July 5, 2024 appears to hold that title, at least over the past half-century or so, at 1.66 ms shorter.
However, based on the data presented by TimeandDate.com, July 10, 2025 is the fifth shortest day, going back to 1973.
This, in of itself, is rather odd. From 1973 through 2019, each year has seen an average length of day greater than usual, and the tally of all that ‘extra time’ at the end of each year has added up to anywhere from 100 to 1,100 milliseconds. In fact, this is the reason why we have introduced leap seconds into our calendar.