For most of recorded human history, time has been carved up into various numbers of days, months, and years. Some ancient cultures relied on the moon to note the passage of days, and this ancient tradition still impacts the way we talk about the calendar (the words “moon” and “month” are actually related). Eventually, mathematicians and astronomers encouraged counting the days using another prominent feature of Earth’s sky — the sun.
Over the course of a few millennia, the calendar has been shaped and rearranged to fit fleeting political whims, religious observances, bureaucratic challenges, and bizarre superstitions. The story of the calendar is the story of humanity, and the answers to these questions show why.
At its start in the eighth century BCE, Rome used a 10-month calendar traditionally believed to be created by its legendary wolf-suckling founder, Romulus. This was a lunar calendar: The beginning of a month, or a new moon, was called the “kalends,” while a waxing half-moon around the seventh of the month was called the “nones,” and a full moon around the 15th of the month was called the “ides.” In this calendar, the year started with March, ended in December, and only added up to about 304 days. So what happened to the 60 or so days between December and March? Well, nothing — Romans just waited for the first new moon before the spring equinox to start the new year, meaning that much of the winter passed in a period without a calendar.
This system, understandably, didn’t work well, and was soon reformed by Rome’s second king, Numa Pompilius, around 713 BCE. Pompilius added additional months — now called January and February — to the end of the year, creating a 12-month calendar (they eventually moved to the front of the year by 450 BCE). The months totaled 354 days, but because of a Roman superstition around even numbers, an extra day was added to January. Since 355 days is still out of sync with the solar year and thus the seasons and celestial events, the king then added extra days, called intercalation, to the latter part of February in certain years. This made the Roman calendar’s average length 366.25 days long — still off, but much better than Romulus' temporal train wreck.
Pompilius’ creation was eventually undermined by Roman pontifices, or priests, who wielded intercalation like a political cudgel — extending the rule of favored politicians while curtailing the term limits of enemies. After 700 years, the Roman calendar was a mess, and the powerful general and statesman Julius Caesar decided to fix it. Following consultation with Rome’s greatest mathematicians and astronomers, he implemented the Julian calendar in 45 BCE. Influenced by the 365-day Egyptian calendar and the mathematics of the Ancient Greeks, this calendar discarded Pompilius’ even number superstition and added extra days equaling 365. But the most notable advancement of Caesar’s calendar was that it embraced the sun as the basis of the calendar rather than the moon. Finally, after 700 long, horribly mismanaged years, the calendar was divided into our modern 12 months.
The short answer is Rome, but the long answer is much more interesting. Remember Romulus’ 10-month calendar? Well, September, October, November, and December simply mean “seventh month,” “eighth month,” “ninth month,” and “tenth month” in Latin, respectively. But these names no longer made sense after the later additions of January, named after the Roman god Janus, and February, named after the Roman purification festival Februa. As for the rest of the months, March is named for the Roman god Mars, April after the Greek goddess Aphrodite (though there’s some debate about whether it might be based on the Latin word aperio, which means “I open” in relation to spring flowers), May after the Greek deity Maia, and June in honor of the powerful Roman goddess Juno.
The names of the last two months come from a few powerful Romans who got a little full of themselves. In 44 BCE, the month Quintilis (which means “fifth” in Latin) was changed to July in honor of Julius Caesar. His heir, Augustus, received the same honor in 8 BCE, when Sextilis (you guessed it, meaning “sixth” in Latin) was changed to August.
February has fewer days because of the superstitions of ancient Rome. In the late eighth century BCE, Romans — including their king Numa Pompilius — held a superstition that even numbers were somehow unlucky. Although he created a version of a 12-month calendar, Pompilius realized there was no mathematical way for every month to have an odd number of days and for the total number of days in the year to also be odd. So while the other months were either 29 or 31 days long, February became the unlucky month to have only 28 days, making Pompilius’ calendar the apparently-less-scary number of 355.
In 45 BCE, Caesar — disregarding Pompilius’ fear of even numbers — added days to a number of other months, but not February. Some experts believe Caesar didn’t want to disrupt the important festivals that took place in that month and so he just let it be. But with the introduction of the Julian calendar, February did receive a consolation prize in the form of an additional day every four years. Speaking of which …
A year isn’t 365 days, it’s actually 365.24219 days. Because of our planet’s frustratingly imperfect solar orbit, calendars need small adjustments as the years pass to keep in alignment with equinoxes and solstices. Ancient astronomers and mathematicians figured that waiting four years and then adding a day made the most sense. In 45 BCE, Julius Caesar introduced the modern leap year, which added an extra day in February every four years (though originally that extra day was added between the 23rd and the 24th). This moved the calendar closer to solar reality at 365.25 days. Close, but not close enough — which is where the pope comes in.
In 1582, Pope Gregory XIII had a problem. As head of the Catholic Church, he realized that Easter — his religion’s holiest day — had drifted 10 days off in relation to the spring equinox, which is supposed to be used to calculate Easter day. That’s because Caesar’s small mathematical error had grown exponentially larger when stretched across 1,600 years. Gregory XIII needed a very slight adjustment to the calendar, just enough to nudge it closer to that magical 365.24219 number. First, Gregory XIII lopped 10 days off the calendar to set things straight, then tweaked the leap year. Now, whenever a new century began that wasn’t divisible by 400 (i.e. 1700, 1800, 1900), no extra day was added. This edged things just enough in the right direction that this new calendar, named the Gregorian calendar, was now 365.2425 days long — close enough. Catholic nations adopted this new calendar immediately, but the Protestant British Empire, along with its American colonies, didn’t sign on until 1752. Today, the Gregorian calendar is used in nearly every country.
Before the invention of A.D. (“anno domini,” which means “in the year of our Lord”) and B.C. (“before Christ”), years were often tracked by the reigns of pharaohs, kings, and emperors. In a way, B.C. and A.D. still reflect this system but focus on just one moment — the birth of Jesus. It’s difficult to trace the exact origins of this system, but one of the earliest recorded uses of “anno domini” occurs in 525 with the work of Dionysius Exiguus, a monk who was trying to determine what days Easter would fall in future years. Crucially, he started his tables with the year 532, stating that this year was “from the incarnation of our Lord Jesus Christ.”
The conception of “B.C.” is slightly murkier. Some believe the Venerable Bede, the famous medieval English historian, was the first to use it, or at least greatly popularized it in his 731 work, Ecclesiastical History of the English People. Others point to a 1627 work by a French Jesuit who used “ante Christum” to describe the pre-Jesus years. The terminology became more widespread during the reign of the Holy Roman Emperor Charlemagne, who used it as a standard form of dating across Europe in the ninth century.
Within the last few decades, more publications and organizations have opted to strip the years of their religious connotation, preferring BCE (Before the Common Era) and CE (Common Era) over the traditional B.C./A.D. system, although the move is not without some controversy. But this subtle change in phrasing doesn’t alter the fact that the world still counts the years in accordance with the birth of Jesus.
The seven-day week is a timekeeping oddity. Unlike days, months, and years, the week doesn’t align with any celestial reality, and it doesn’t divide elegantly into existing periods of time. For example, there aren't 52 weeks in an average year — there are 52.1428571429. So how did this happen? Babylonians, the ancient superpower of Mesopotamia, put a lot of stock in the number seven thanks to the seven observable celestial bodies in the night sky — the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. This formed the seven-day week, which was adopted by the Jewish people, who were captives of the Babylonians in the sixth century BCE. Eventually, it spread to ancient Greece and elsewhere thanks to the battle-happy Macedonian Alexander the Great. Efforts have been made throughout history to reform the seven-day week, but this oddball unit of time has become ingrained in many religions, including Judaism, Christianity, and Islam, rendering any sort of tweak pretty unlikely.