We think of a year as the amount of time it takes for the Earth to revolve around the Sun once. Pretty simple, right?
It gets complicated when you try to break it down into smaller units: days, or the length of time it takes the Earth to complete one rotation on its axis. A solar year is 365.242159 days long.
That's where leap days come in.
This year's leap day, Feb. 29, 2016, may mean that someone born on Feb. 29, 1916, could jokingly claim they're turning 25-years-old rather than 100. But this Monday, celebrated with a Google Doodle, represents much more than jest.
Leap days make our Gregorian calendar more accurate. But adding a day here and there hasn't been the easiest fix.
When the Romans decided to make a calendar based on the solar year, and therefore the seasons, rather than lunar cycles, they had their work cut out for them.
Julius Caesar sought to create a more standardized calendar in line with the seasons, so, for example, people would know when to hold religious festivals. Under the guidance of Alexandrian astronomers, thanks to his visits to Egypt, he devised the Julian Calendar.
The Julian Calendar had 365 days in a year with an extra day tacked on to every fourth year.
But Julius Caesar overcompensated. With .242159 extra days in a year, a whole extra day every four years added too much.
By 1582, the calendar was off by 10 days.
We have Pope Gregory XIII to thank for our current way of determining leap days. Keen on celebrating Easter close to the spring equinox every year, the Pope took matters into his own hands and revised the calendar.
The Gregorian calendar, the most widely used today, has a leap day every four years except on centennial years. But, to make matters a bit more complex, any centennial year that is divisible by 400 does have a leap day. So 1996 was a leap year, 2000 was a leap year, but 1900 was not.
"That gets us closer to the real orbital period," Ken Seidelmann, an astronomer at the University of Virginia, tells The Christian Science Monitor in an interview. But, "it's not a perfect fit."
Still, "There won't need to be an adjustment for a good long time period."
But that hasn't stopped some people from proposing more permanent calendars.
These calendars have arisen from the business world. Right now, financial quarters do not all have the same number of days in them.
"It would be much easier if the interval of time over which we do business was segmented equally," Anthony Aveni, an astronomy professor at Colgate University, who wrote the book "Empires of Time" on the history of timekeeping, tells the Monitor.
Steve Hanke, an economist at Johns Hopkins University, says having an even number of days in a quarter would simplify financial calculations.
Dr. Hanke and his colleague Richard Henry have proposed their own new calendar: The Hanke-Henry Permanent Calendar. In that calendar, every year would have 364 days and a leap week would be tacked on every five or six years to make make up for the missing parts of a solar year.
One advantage of this calendar, Hanke tells the Monitor, is that the date would fall on the same day every year. This would cut down on time spent revising schedules for companies, universities, and other institutions each year.
In any scenario, it doesn't look like we'll be doing away with any sort of leap unit anytime soon. But that probably isn't such a bad thing. If Caesar hadn't started it all by including leap days in his new calendar in 46 BC, today would actually be in the middle of July 2017.
If you're curious to see how difficult these calculations are for yourself, check out the Los Angeles Times' interactive graphics depicting leap days.