It is a strange thought to consider: what if, right now, it was simultaneously ten days ago in Alaska? Such that if you dispatched a postcard to Juneau, it would likely, in the eyes of the recipient, arrive from the future? Or, to ask an Englishman what happened on the 11th of September, 1752 only to have him respond—truthfully— that the day never happened?

We tend to take it for granted that most everyone in the modern world can agree, in spite of confusion over time zones, what day it is. We tend to see history as a patterned unfolding of events on dates in uniform, unambiguous years going back to the time of the Ancients. Though Elizabeth I was born in the 16th century, we think of that particular birthdate—September 7— as analogous to our 21st century September sevenths.

But this isn’t really accurate.

Our modern notion of date—the Gregorian calendar—began as a problem with Easter in 16th-century Italy. The Julian calendar had been used widely and consistently since the time of its eponymous creator, Julius Caesar, in 46BC. But the moon seemed to be drifting. No longer was the vernal equinox falling quite when it should, date-wise, and the antique lunar calculations for when Easter should be left church leaders with a 4-day moon lag to contend with. The whole machine needed some adjustment to come back in alignment with the edict from the Nicaean council of 325AD: that all Christians must celebrate Easter on the same, unequivocal date.

A Calabrian doctor, Aloysius Lilius, cooked up a solution. He tinkered with a bit of math and the era’s better ability to approximate the exact length of an earth year. His new system, graciously named after the seated pope, Gregory XIII, is the calendar we use to this day. A supporting papal bull was dispatched in February, 1582.

This worked out okay, immediately, for Catholic nations. Spain and Portugal got on the bus right away. France lagged slightly but didn’t require too much cajoling. Protestant countries, however, eyed Rome suspiciously, wary of a popish plot. It wasn’t until over a hundred years later, in 1700, that Denmark finally agreed, and Britain was all out of whack until 1752. And recall that in 1752, England’s lands included the American colonies. The sun set on the British Empire on September 2, 1752, and rose again—on the 14th. There are tales, perhaps apocryphal, of malcontents rioting and demanding their lost days back.

Notoriously late in the game was Russia, who didn’t budge until after the October Revolution (which, somewhat hilariously, actually took place in November if you use the Gregorian calendar). Thus, during the mid-19th century, Alaska, then their possession, lagged some ten or so days behind the continental United States. It’s hard to say when, exactly, Alaska was sold to the US, as Russians and Americans disagree about the date.

I’ll leave you with one random fact: In the Gregorian calendar, you are more likely to have a leap day land on a Sunday or a Friday than any other day of the week. Figuring out why I’ll leave to you—too much math for yours truly.