The calendar, that tidy, seemingly immutable grid on the wall, is really an arbitrary mash-up of astronomy, religion and petty politics, all laid over the untidy reality that the orbits of the moon around the Earth don’t synchronize with the Earth’s trips around the sun—and that’s why we’re in a leap month right now.

It is so changeable, in fact, that we honor our Founding Fathers on the wrong days.

“We celebrate Thomas Jefferson’s birthday on April 13, 1743, but if you were in the room when he was born the calendar would say April 2,” says Edward Murphy (Grad ’93, ’96), a UVA associate professor of astronomy, because we switched to the Gregorian calendar during his lifetime and dropped 11 days. Thus on Jefferson’s gravestone you’ll see a little OS next to his birth date, meaning Old Style.

More dramatic, we celebrate George Washington’s birthday on Feb. 22, 1732, but in the room the calendar would have read Feb. 11, 1731, because we not only dropped 11 days but New Year’s Day also switched from March 25 to January 1 during his lifetime.

Early cultures watched the stars and observed that it takes a bit more than 365 days for the sun to complete a trip through the constellations, and 29½ days for the moon to run through its cycles. They called the moon cycle a month, and created separate calendars for religious festivals, markets and political events. Some put the start of the year on March 1, some on the Equinox. The first king of Rome imposed a standardized 10-month calendar that began March 1, says John F. Miller, a UVA professor of classics. Some months seemed to be named after gods—March was Martius, after Mars, the god of war—and some were numerical, so the fifth month became Quintilis, the sixth Sextilis, the seventh September, and so on. But 10 months wasn’t getting them through an entire year, so tradition says that it was King Numa who tacked January and February onto the end, making 12 months. Even then they were completing twelve cycles in 355 days—10 short of a celestial year. So every once in a while they’d add days to the last month, which at the time was February.

“We consider the calendar nonchanging,” Murphy says, “but back then if you were a politician who wanted to collect extra taxes you could throw in another month, with the excuse that you were making the calendar align with the seasons.”

In 46 BC, Julius Caesar ordered a major reform, producing a 365-day calendar with an extra day added to February every four years, Miller says. After Caesar’s death, Quintilis was renamed July in his honor. Then Sextilis was renamed August in honor of his successor, Augustus Caesar. By then, months no longer aligned with the cycles of the moon, and July had expanded to 31 days. August only had 30, so the senate stole a day from February to make Augustus’ month equal to Julius’.

It actually takes 365.242190 days for the earth to orbit the sun, and in 1582  Pope Gregory the 13th noticed that the vernal equinox had drifted from March 21 to March 11. If the drift wasn’t corrected, Easter would no longer be a spring holiday, which mattered enormously to the Catholic Church. So in October of that year, he dropped 10 days from the calendar. Thursday, Oct. 4, was followed by Friday, Oct. 15, which led to temporary chaos. While Catholic countries followed Gregory’s reform, Protestant countries refused for more than 100 years later, and Great Britain and her Colonies didn’t make the change until 1752. Hence the difference in Jefferson’s birthday.

Even our current calendar is not perfect; it’s off by 26 seconds from the astronomical year. By the year 4909, the vernal equinox will have slipped by one full day, and the calendar will have to be changed once again.