When it comes to the word ‘leap,’ English has no shortage of expressions. We can leap for joy, grow by leaps and bounds, take a leap of faith or leap at an opportunity. This week, we take a different kind of leap altogether.

Every four years, we tag on an extra day to the end of February and call it a ‘leap year.’ Rather than an idiomatic leap, it is a sort of metaphorical leap into an occasionally available pocket of time.

I have often wondered why we don’t call these years ‘add-on years’ or ‘time-saving years.’ Why leap? According to the Oxford English Dictionary, the name probably refers to the fact that in a leap year, any fixed festival after February does not fall on the next weekday but the following. In other words, we leap over a day. 

If your birthday was on a Monday in 2013, it fell on a Tuesday in 2014 and a Wednesday in 2015. This year, if the date comes after February, it will leap over Thursday and fall on a Friday.

Leap year is the solution to the fact that it takes longer than 365 days for the Earth to make one complete orbit around the sun—it is closer to 365.25. Without leap day, our calendar would not be consistent with the seasons. This was first addressed by the Romans, who were accustomed to adding days and even months to their 10-month calendar at irregular intervals for a variety of reasons. These ‘intercalary’ days and months could be added on religious grounds or even for political purposes (such as allowing elected public officials more time in office).

When Julius Caesar became the High Priest in 46 BC, he implemented the Julian calendar, which had a regular year of 365 days divided into 12 months with an intercalary day once every four years, or leap day.

The day was initially called ante diem bis sextum Kalendas Martias, or ‘sixth day doubled before the first day of March.’ Eventually, it was shortened to bis sextum, or ‘bissextile’ in English. Most Romance languages still refer to leap year as a bissextile year—consider French année bissextile, Italian anno bisestile and Spanish año bisiesto.

The bissextile day continued into the Gregorian calendar. This calendar, however, added additional corrections to fix the conundrum that we were still falling behind—Easter wasn’t matching up with the March equinox. It turns out that Earth’s journey around the sun is actually 365.2422 days, which means that we were still surpassing the solar year by 11 minutes and 14 seconds per year! As a solution, the Gregorian calendar skips a leap year at the beginning of most (but not all) centuries. 

In order to account for additional inconsistencies in time, which come from the world’s axis of rotation, there are also ‘leap seconds,’ which are periodically added to the official world time. In 2012, Google coined the term ‘Leap Smear,’ in which they add a few milliseconds each day to a computer’s time-keeping mechanism as a solution for keeping up with leap seconds.

Traditions around the day vary from country to country. A European tradition states that leap day is the only acceptable time for a woman to propose marriage to a man, and there are several Italian proverbs that suggest that the year should be met with caution. For example, anno bisesto, anno funesto means ‘leap year, doom year’ and anno bisesto tutte le donne senza sesto, ‘in a leap year, women are erratic’—this certainly explains the reason behind their wanting to propose marriage.

Leap-year babies are called ‘leaplings’ or ‘leapers’ and, depending on what country they are born in, their official birthday varies. Some countries, such as the UK and Hong Kong, deem the official leaper birthday to be March 1, while others such New Zealand and Taiwan declare February 28 as the day for celebration. The US has no official rule on the matter.

Leaplings are often frustrated—they report computerized systems that don’t recognize their birthdays, constant confusion at the DMV and pressure to actually change their birthdate. Of course, there are some advantages as well.

One leaper writes about her non-traditional fifth birthday party, which included beer and a stripper; others feel quite justified in celebrating for two consecutive days; and I recently discovered the story of a leaper who died shortly after her 25th birthday. For those of us born in a common year, this would be considered a tragedy, but in her case, 25 was a true accomplishment!