The world transitioning from one quarter century to the next on New Year’s Day 2026 may be a nice bit of trivia in itself.
As the clock chimes midnight on January 1, 2026, exactly 9131 days will have passed since the beginning of the century on January 1, 2001.
But strictly speaking, that’s not quite true for some island dwellers in the South Pacific Ocean. A lot has changed in the world of time and date in the past 25 years, and that includes Samoa and Tokelau redrawing the International Date Line. In 2011, the Pacific islands switched from the east of the line to the west, effectively skipping December 30.
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In the same way, they will have one day less overall this century. This means that, based on the number of hours, they will reach the quarter-century mark at 18:00:00 local time on January 1.
Of course, the same could be said about people who have moved to a different time zone since 2001. But who’s splitting hairs? Well, if you insist: if we’re counting seconds, we don’t yet know when the quarter-way point of the century will fall.
On the clock, the first 25 years of the 21st century equal precisely 788,918,400 seconds. However, Earth’s spin has slowed down ever so slightly over the years—at least up until recently. That’s why a total of five leap seconds have been inserted since 2001: the first one in 2005 and the last one in 2016.
2025 had the shortest days since atomic clocks were invented. Here’s the catch: Earth’s rotational speed varies in ways that are difficult to predict. So we can’t say for sure how many more leap seconds will be added before the end of the century. But there are also proposals to change the “rules” for leap seconds. If these are adopted, it would mean we wouldn’t need any leap seconds for at least a hundred years. In this case, the quarter-way point of the century in terms of seconds would be 3.75 seconds before the stroke of midnight on January 1, 2026.
The modern Gregorian calendar is quite accurate in reflecting the true length of a year, as defined by the average time it takes for the seasons to repeat as the Earth completes roughly a full orbit around the Sun. This is called a tropical year. But it’s not 100% accurate, and a quarter century is a perfectly sized time span to illustrate that.
Our calendar is off by one day every 3236 years or so—or about 27 seconds per year. Even with leap days taken into account, an average calendar year is very slightly longer than an average tropical year, so each year, seasons begin a tiny bit earlier on the calendar than the previous year. Since early 2001, Earth has outpaced our calendars by roughly 11 minutes. So, based on the calendar, seasonal events like the March equinox will happen about 11 minutes earlier on average than they did in 2001.
So why is it claimed that the first 25 years of the century will have passed on New Year’s Day 2026—instead of 2025? It’s not a mistake, and it reveals another quirk of humankind’s timekeeping system.
Well, the Gregorian year count started with year 1, and there was never a year zero. That’s why the century didn’t start in the year 2000, but a year later, on January 1, 2001. As a result, the century’s first 25 years will have elapsed on New Year’s Day 2026.
If asked what year marked the halfway point of the decade, "official" timekeepers would actually pick 2025.
There’s no right or wrong answer here. But, purely based on how we talk about time spans, decades can be said to start with round years, while centuries and millennia don’t.