No one alive has seen spring come as early as it will this year, due to a complex combination of natural and man-made occurrences. This year, the vernal equinox, which signals the beginning of spring, occurs at 11:50 p.m. Eastern Daylight Time on Thursday, March 19.

The last time the vernal equinox occurred this early was 124 years ago. Whether the first day of spring occurs on March 21, March 20 or March 19 has to do with leap years, century marks and the length of time it actually takes the Earth to make a trip around the sun.

Sharpen your math pencil. Or, better yet, take out your phone and use the calculator app. This is the 21st century, and that changed everything.

Space.com says that deep at the root of this riddle of 2020’s early spring is the fact the number of days in years and seasons aren’t even. Neither can be divided in equal portions like a pie, so simple division doesn’t work to reconcile the natural world astronomical calendar and human-created calendar we use to mark time.

Things going on in space that have to do with how the Earth’s elliptical orbit pattern, its axial tilt and the gravitational pull of the other planets all affect the equinox and solstice dates, and Space.com has a detailed explanation if you’re interested.

When the math is worked out, it takes the Earth 365.25 days to make its orbit around sun. On the astronomical calendar, each of the four seasons includes a fraction of a day. There are 92.758 days in spring, 93.651 in summer, 89.842 in autumn and 88.994 days in winter.

Those troublesome fractions necessitated human-created calendar resets known as leap years — though it’s not as simple as adding a Feb. 29 every four years. Math involving the intricacies of the universe is anything but simple.

But without the quadrennial leap years, and harder resets that occur in century years, the seasons would arrive earlier and earlier.

To further understand why spring 2020 is the earliest since 1896, go back to 46 B.C. Acting on the advice of Alexandrian astronomer Sosigenes, Julius Caesar added leap days every four years to the Julian calendar he was developing.

The addition of leap years every four years keeps the natural astronomical and human-created calendars in sync. They’re pretty close, but not exact, and that’s when things get even more complicated.

As The Old Farmer’s Almanac outlines it:

Years that are divisible by 4 are leap years and have 366 days instead of 365. But if that year is divisible by 100, as most new century years are (1700, 1800 and 1900, for example), Feb. 29 isn’t added. Skipping leap years on century years seemed to do the trick and push the date of each season change to the 21st.

But as each century grows older, equinoxes and solstices occur later to the point the seasonal changes may slip ahead to 20th and, as the vernal equinox is in 2020, the 19th.

Every four centuries, our human-created calendar is further adjusted so the season changes don’t keep creeping earlier and earlier.

That happened in 2000, both a leap year and a century year. So shouldn’t the calendar czars have skipped leap year? No. When leap years and century years occur simultaneously on a year that is divisible by 400, the Feb. 29 leap day is added.