25 years ago, with the transition from December 31, 1999 to January 1, 2000, almost everyone celebrated the end of the 20th century and the beginning of the 21st – as well as the beginning of a new millennium. Therefore, 2025 marks the end of the first quarter of the 21st century. But is this really the case?

As early as the late 1990s, mathematicians had expressed their disagreements: the beginning of the third millennium and therefore the 21st century should be celebrated on January 1, 2001. Why? Because there was never a year 0 – after the year 1 BC came the year 1 AD. Therefore the first 2000 years ended with the end of 12/31/2000 – just as a man celebrates his 30th birthday when he has completed his 30th year of life.

Billions celebrated the wrong day

Nevertheless, the world… had made up its mind. As 1999 came to a close, billions of people around the world celebrated the beginning of a new era, a new century, a new millennium. The same also happened in the past: in the 1700s, 1800s and 1900s, people were also convinced that they were entering a new century.

The 20th century was a century of terror, of two World Wars, the Holocaust, the atomic bomb, the Cold War, environmental destruction and humanitarian disasters. Between 1900 and 2000, however, there were also many innovations and new inventions, such as radio, television and the internet. So far, the 21st century has also brought new developments, especially in the field of technology, such as smartphones and streaming services – but with them, new wars broke out again, while humanity was faced with other disasters, as well as climate change.

What do mathematicians say?

But back to our topic: does the first quarter of the 21st century end with the end of 2024?

“For all those who celebrated the new millennium after December 31, 2000, 2025 does not mark the beginning of the second quarter of the century,” says Daniel Potts, a mathematician at Chemnitz University of Technology. But they are probably very few. “For all those who celebrated the turn of the millennium with the end of 1999, the first quarter of the 21st century now comes to an end,” states Potts. “It all depends on how we define the beginning of a century.”

A quarter of a century lasts 25 years. “But how long is a year?” Potts asks, pointing out that there are also leap years. The duration of a century, but also of a quarter of it, therefore also depends on how many leap years there are in this interval.

When exactly does the first quarter century end?

“According to a time calculator, there are exactly 36,525 days between January 1, 2000 and December 31, 2099. So, a quarter of a century is completed after 9,131.25 days.” On December 31, 2024, however, 9,132 days have passed. Therefore to be… perfectly mathematically accurate, one would have to celebrate the completion of the first quarter of the 21st century at 06:00 in the morning on New Year’s Eve. But as Professor Potts says: “That’s something even mathematicians probably won’t do.”

Edited by: Giorgos Passas