LEP · Now boarding
When is the next leap day?
Leap day falls on ….
Counted in your own time zone, —.
Leap days fall on 29 February in years divisible by four — except centuries, unless those are divisible by 400. The counter works out the next one properly.
Not simply every four years
The rule most people know is only the first third of it. A year divisible by four is a leap year, unless it is divisible by 100, in which case it is not, unless it is also divisible by 400, in which case it is. So 2000 had a leap day, 1900 did not, and 2100 will not — meaning there is an eight-year gap coming that catches out a great deal of software.
Why the exceptions exist
A year is about 365.2422 days. Adding one day every four years overshoots slightly, so the century rule removes three leap days every four hundred years to pull the calendar back. What is left drifts by roughly one day in three thousand years, which was considered good enough in 1582 and has been ever since.
The date that mostly is not there
Being born on 29 February gives you a birthday in roughly one year in four, and the rest of the time you choose between the 28th and 1 March. It is the only date on the calendar you can genuinely count down to for eight years, which makes it the purest test of whether you like counting down to things.
Questions
When is the next leap day?
The counter above works it out from today using the full leap year rule, so it is always the next real one.
Is every fourth year a leap year?
Almost. Century years are not, unless divisible by 400 — so 2000 was a leap year and 1900 and 2100 are not.
Why do we have leap days?
A year is about 365.2422 days long. Without an extra day roughly every four years the calendar would drift against the seasons.