How Leap Years Work

Every time you flip a calendar, a year seems to be a tidy 365 days. But in reality, it takes Earth 365 days and roughly 6 hours to complete its orbit and return to the same season. There is always a little time left over.

Our everyday calendar rounds a year down to 365 days for convenience. This means our calendar ticks about six hours faster than Earth's real journey each year. A six-hour gap feels minor at first. Yet after two years, it becomes half a day (12 hours), and after four years, a full 24-hour day drifts away.

To close this gap, humanity decided to add a bonus day to February every four years. Adding February 29 brings the calendar back in lockstep with Earth. We call this extra day a leap day, and a year with 366 days is known as a leap year.

Simply adding a day every four years causes another headache. To be exact, a solar year is not precisely 365 days and 6 hours; it is about 365 days, 5 hours, 48 minutes, and 46 seconds. That is about 11 minutes and 14 seconds shorter than 6 hours.

Adding a full 24 hours every four years overcompensates, making the calendar fall behind by about 45 minutes every four years. Over four centuries, this tiny discrepancy adds up to roughly three full days. Left unchecked, the calendar would run noticeably too slow. In fact, after centuries of using this simpler system (the Julian calendar), Pope Gregory XIII had to skip 10 days in 1582 to fix the drift.

That is why our modern Gregorian calendar relies on a clever three-step filter. A year is a leap year if it is divisible by 4, but century years divisible by 100 (like 1900 or 2100) are skipped and kept as common years. However, century years divisible by 400 (like 2000 or 2400) stay leap years after all. Thanks to this fine-tuning, our calendar drifts by only about one day every few thousand years.

A day or two might not seem like a big deal in daily life. Over centuries, however, that tiny mismatch causes a massive drift across seasons.

Without leap years, the calendar would drift away from the solar cycle by roughly 24 days every century. After about 700 years, the calendar would be off by six full months. July would bring freezing blizzards in the Northern Hemisphere, while January would bring sweltering summer heat. For human societies relying on regular seasons for planting and harvesting, keeping the calendar synced with the sun was vital.

That quiet February 29 on our calendar is an ingenious human adjustment mechanism, designed to keep the clocks of human civilization perfectly synchronized with Earth's voyage through space.