In a few days time, a good friend of mine will celebrate his seventh birthday.

For those who haven’t caught on, let me explain. My friend was born in 1992, so he’s in his late twenties. But 1992 was a leap year, and he was born on that added day, February 29, which we get in a solar calendar leap year. This means he has only been technically been able to have a birthday once every four years (although I believe when he was younger his parties were held on the 28th, in a triumph of emotion over logic.)

So much for the Gregorian calendar leap year. But this got me thinking. What about Jewish leap years?

As many of you will probably already be aware, in the Hebrew calendar a leap year contains an extra month rather than an extra day. This is because the lunar calendar, which is used in Judaism, has approximately eleven fewer days than the solar calendar, and it’s necessary to have this extra month every few years to balance things out in order to ensure that all our Jewish festivals stay in roughly the same place during the year and we don’t end up, for example, with Rosh Hashanah in December or Chanukah in June.

So far, so commonly known. But until recently, I had not considered how this system works in practice.

When I was younger, the biblical concept of a shemitah year —an ancient idea seemingly unique to Judaism, where the fields are left to lie fallow — was always translated as a “leap year”. But that’s not an accurate translation. A shemitah year occurs just once every seven years. Jewish leap years occur far more frequently than that — seven years out of every 19 will have an extra month of Adar. The first Adar, Adar Rishon, is considered to be the extra month, rather than the second, Adar Sheni. That’s why in leap years the festival of Purim is celebrated in the second Adar rather than the first.

To be exact, the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteenth years of that 19 year cycle will contain an extra month. In Hebrew, such a year is called a Shana Me’uberet, or “pregnant year”.

The 19 year rotation is known as the Metonic cycle, after Meton of Athens, a Greek astronomer in the 5th century BCE, who calculated that 19 solar years and 235 lunar months were almost identical in length (give or take a few hours). Evidence suggests, however, that among the Babylonians this system existed long before that.

A rabbinic teaching says, “If a person tells you there is wisdom among the nations, believe them.” And this appears to be a prime example of such wisdom being embraced by the sages. As the Torah describes, the original Jewish calendar system was based around sightings of the new moon, with the Sanhedrin declaring the new month when witnesses could be successfully established to have looked for and seen the crescent in the night sky. Under that system, an extra month would be added once every two or three years to maintain the lunar year roughly equal to the solar year.

However, this system would begin to shift in the centuries after the destruction of the Second Temple in 70 CE. The potential for a conflict where one great rabbi accepted a sighting of a new moon while another did not may well have been one of the reasons for this change; the mishnah famously describes just such a narrowly averted schism between Rabban Gamliel and Rabbi Yehoshua, the two most senior sages of their generation. They disagreed over the first day of the month of Tishrei and that difference of opinion meant a difference in their calculations of the date of Yom Kippur. Rabban Gamliel, holding the more senior position of the two, demanded that on the day his colleague had calculated to be the Day of Atonement, Rabbi Yehoshua should appear in front of him with a staff and money belt (strictly forbidden on Yom Kippur). Having agonised over the decision, Rabbi Yehoshua finally decided to submit to Rabban Gamliel’s authority, and on that occasion, a crisis was averted.

But the incident must have rung alarm bells for the sages, because in the centuries afterwards the calendar system began to transform into the method we have today, most famously being laid out in detail around 840 years ago by Maimonides in his compendium of Jewish law, the Mishnah Torah.

In other words, with its shift from gazing at the heavens for the moon to a strict calendar system, the story of the Jewish calendar system can be summed up as follows; look before you leap. And happy birthday to my friend.

I’m thinking about getting him a very nice bottle of wine —although technically he should wait until his 18th birthday in 2064 before drinking it.)