Leap Years, Leap Seconds, and Why Calendars Drift

Earth orbits the sun in about 365.2422 days. That leftover quarter-day is the villain of this story: ignore it, and your calendar drifts against the seasons until August snows in the northern hemisphere. Everything from leap years to the Gregorian reform is an attempt to bribe that fraction into behaving.

Julius Caesar's good-enough fix

In 45 BC the Julian calendar introduced a leap day every four years — treating the year as exactly 365.25 days. Close, but about 11 minutes too long. Eleven minutes a year compounds: by the 16th century the calendar had slipped roughly ten days against the equinoxes, dragging Easter away from its springtime anchor.

The ten days that vanished

Pope Gregory XIII's reform of 1582 did two things: it deleted the accumulated error — Thursday 4 October was followed directly by Friday 15 October — and it refined the rule. Century years would no longer be leap years unless divisible by 400. So 1600 and 2000 kept their leap day; 1700, 1800 and 1900 did not. The average year became 365.2425 days: off by only about one day in 3,000 years. Protestant Britain held out until 1752, by which point it had to drop eleven days (2 September was followed by 14 September). You can test any year against the rule with our leap year calculator.

The leap second's short life

Atomic clocks created the opposite problem: our timekeeping became steadier than the Earth itself, whose rotation wobbles slightly. From 1972, occasional leap seconds were inserted into UTC to keep it within 0.9 s of Earth-rotation time — 27 of them, the last on 31 December 2016. But leap seconds crash software in ways leap years never did, and in 2022 the world's metrology bodies voted to stop adding them by 2035.

So spare a thought for anyone born on 29 February — and if you want to know exactly how many days you've been alive through all those leap years, the age calculator has you covered.