Leap year 2024: Why do we have an extra day this February?

A UAB physics expert breaks down the details and history of why we recognize the Gregorian calendar leap year.

Stream Leap YearA UAB physics expert breaks down the details and history of why we recognize the Gregorian calendar leap year. Every four years, we observe a leap year, because of which, February will have an extra day this year. Younas Khan, Ph.D., University of Alabama at Birmingham instructor in the Department of Physics, explains the science and history behind why we take part in this practice.

“We know a calendar year as having 365 days; however, in reality the Earth rotates around the sun in roughly 365.24 days,” Khan said. “That .24 is essentially just shy of a fourth of a day; but as years go by, those fourths of a day add up.”

Without a leap year system in place, the accrual of these portions would eventually cause the seasons to completely drift from their accustomed alignment. To maintain the synchronous timetable of the seasonal year, there is an additional calendar day added to February every four years.

“Summer starts around June,” Khan said, “but without the leap years, after a few hundred years we will have summer in November.”

The desynchronization of the seasons could, in a worst-case scenario, cause an upheaval of the economy due to the impact it would have on the universal farming system of crops, ultimately impacting supply-and-demand trends.

The leap year has been incorporated since the era of Julius Caesar; however, in 1582, the Julian calendar was modified by Pope Gregory XIII into what we currently, and widely, use — the Gregorian calendar.

While the addition of a day every four years would seem to solve this problem, the addition of one day does not quantify the total of the four accumulated .24s of a day. Due to the smaller, yet still evident, overcompensation, the Gregorian calendar balances this calculation by not adding a leap day to century years.

“The only exception to skipping leap days at the start of a century is when that century year is divisible by 400,” Khan said. “The last time we saw this exception was in the year 2000, which should have been a skip; but it was in fact divisible by 400.”

For example, the years 2100, 2200 and 2300 are not leap years, as they are not divisible by 400, while 2400 will be a leap year.

While the numbers are miniscule discrepancies, over the course of centuries these accumulations could have a great impact. For example, prior to the Gregorian modification, the Julian calendar drifted off course by one day every 128 years. 

Khan adds that, while it is customary to add the extra day to February, the additional day is not contingent upon where in the year it falls at all.

“A common misconception is that the additional day must be added to February,” Khan said. “The day can be added to any month of the year. It is just convenient to add it to February since it is already the shortest month of the year.”