It’s 2021, and that means people across the United States are looking at new calendars. But have you ever wondered how January got its name or why each year has 365 days?

The word “January” comes from the Roman god of beginnings, Janus. The idea of a “month” comes from lunar (moon) phases, which tend to last about 29 or 30 days. Weeks are seven days, because that was the closest the ancients could come to breaking up the moon cycle into four equal parts. We can thank the ancient Babylonians for that, as well as the names of the days of the week, which correspond to their seven, “sacred” celestial bodies. And the notion of a “year” is based on how long it takes the Earth to complete an orbit around the sun.

But technically it takes the Earth 365.24219 days to complete its orbit around the sun each year, not exactly 365 days. And this inconvenient remainder has been throwing the calendar off for centuries. For instance, the 2021 calendar on your wall is short by 5.8 hours.

The problem is mostly fixed by adding an extra day to the month of February every four years. These are called leap years, and the last one took place in 2020. One way to think about it is that the leap year system allows all those troublesome partial days to add up for three years and then groups them together to create a February 29.

This idea hails back to the time of Julius Caesar, leader of the Roman Empire, in 46 B.C. But even the leap year is slightly wrong. Adding a whole day every four years is actually a smidgen too much. And while we’re talking about a difference of minutes, time has a way of adding up through the centuries.

So much so, that by the time the 1570s rolled around, the calendar Caesar had instituted had slipped off-course by about 10 days. This was concerning to Pope Gregory XIII, who wanted to make sure that Catholics celebrated sacred holidays on the proper days. So he created a commission to find a way to “fix time.”

Of course, time is not easily fixed, but after five years of tinkering, Pope Gregory’s commission recommended deleting those 10 extra days from the calendar. Then, to keep everything on track for the future, they decided that leap years would get skipped on centennial years that are not divisible by 400. In other words, there would be a leap year in the year 2000, but not in 1900, and so on.

Confusing, right? No wonder it took the commission five years to figure it out. This is why the current calendar system is known as the Gregorian calendar. And it still isn’t 100 percent accurate! In fact, by the year 4909, our calendar will be off by a whole day. But that’s a problem for your great-great-great-great-great-great-great grandkids to solve.

Bittel is a freelance journalist who often writes about animals. His children’s book, “How to Talk to a Tiger . . . and Other Animals,” will be published in April.