How many days
until your birthday?

In 2014, behavioral scientists Adam Alter and Hal Hershfield published a study in the Proceedings of the National Academy of Sciences that examined why people make unusually large life decisions in the year before a round-number birthday. They coined the term "9-enders" — people who are 29, 39, 49, 59, and so on — and found that this group disproportionately ran first marathons, had extramarital affairs, made major career changes, and rated their lives as either meaningfully more or less satisfying than they had previously.

The mechanism is what psychologists call decade reasoning. Humans use 10 as a natural checkpoint — a round number that triggers a stock-take of where we are versus where we thought we would be. The year before 30 is experienced as the last chance to accomplish the things a 20-something "should" have done. The year before 40 carries the accumulated weight of decisions made in the 30s.

Interestingly, the same research found that 9-enders were also more likely to seek meaning — to sign up for therapy, to call estranged family members, to travel to places they had always said they would visit. The approaching decade marker acts as a motivational trigger that can push people toward both positive and negative extremes.

Knowing this about yourself is genuinely useful. If you are approaching a round-number birthday, the existential restlessness you may be feeling is a documented psychological phenomenon, not a sign that something is wrong. Many people use that energy deliberately — treating the year before the milestone as a window for intentional change rather than anxious reaction.

The birthday as a personal celebration is a surprisingly recent and culturally specific concept. In ancient times, only kings and gods had birthdays worth marking — the average person's birth date went unrecorded and uncelebrated. The shift toward individual birthday celebrations in Western culture accelerated in the 19th century alongside the spread of civil birth registration, which gave ordinary people an official date to point to.

Germany is considered the origin of many modern birthday customs. The Kinderfeste (children's festival) tradition, dating to the late 18th century, combined cake, candles, and gifts into a recognizable template. The specific practice of putting candles on a cake — one for each year of life, plus one "to grow on" — is documented in Germany from at least the 1700s.

The United Kingdom has a tradition of sending a birthday card from the monarch to anyone who reaches 100 years old, and again for every year thereafter. The practice began with King George V in 1917 and has been maintained by every subsequent monarch. The Centenarian team at Buckingham Palace currently sends around 12,000 cards per year — a number that has grown steadily as life expectancy rises.

Leap day birthdays — February 29 — are among the rarest possible birthdays, shared by approximately one in 1,461 people (roughly 0.07% of the population). People born on this date are called "leaplings" or "leapers." In non-leap years, the question of when to celebrate divides countries: in the UK, the legal birthday is March 1; in New Zealand and parts of Europe, it is February 28.