You might’ve heard there are 365 days in a year. Wrong! Each day on Earth is made by our planet’s rotation on its axis, bringing the sun into view in the morning and causing it to slide into the opposite horizon each evening. And as we spin, so do we orbit around the sun, once every 365.25 days. That means that every four years, those quarter days—roughly 6 extra hours per year—add up to one full day that has to be added to the calendar.
If leap days weren’t added, in a few hundred years the monsoon would start in January and the desert would be chilly in July.
The system of leap years, devised originally in the year 46 BC, isn’t perfect. The extra time in Earth’s annual orbit is actually 5 hours, 46 minutes and 48 seconds—not a full 6 hours. In 1582 it was revised so that there is no leap day in century years that aren’t evenly divisible by 400. So 2000 was a leap year, but 1800 and 1900 were not, and 2100 won’t be.
For most of us that’s just complicated math. But to anyone born on Feb. 29, it’s a harsh reality. Saturday will be their first birthday in four years.