Mark Anderson, a Durango resident of almost 50 years, was looking forward to finally becoming an adult Saturday. Anderson, whose birthday falls on Feb. 29, was 72 – turning 18 this year.

Those with Leap Day birthdays, called leaplings by some, spent the last week of February gearing up for celebrations held only once every four years.

Leap Day comes around every four years thanks to the quirks of the Gregorian calendar, established in the 16th century and still used in many places around the world. The extra 24 hours of Leap Day are built into the calendar to ensure that it stays in line with Earth’s movement around the sun.

Those with Feb. 29 birthdays are part of an exclusive club: The odds of a Leap Day birthday are 1 in 1,461.

Anderson’s week of preparation involved sitting in the sun, “enjoying life” and plotting ways to get friends to buy him a birthday breakfast. Friends were already buying him lunch and dinner, he said.

“I’m going to be fat and happy,” he said about his birthday plans.

Only about 5 million people in the world today have been born on Feb. 29. Among those are famous figures such as motivational speaker Tony Robbins (born in 1960) and hip-hop artist Ja Rule (born in 1976).

Most people say a year is 365 days, but really it is about 365.2421 days, according to History.com.

In 45 BC, Julius Caesar copied the Egyptian solar calendar, which featured 365 days and an occasional leap month. Caesar calculated that a year lasted 365.25 days so he just added an extra day every four years.

But his calculations were 11 minutes off, and by the 16th century, the calendar was off by 10 days.

Pope Gregory XIII fixed the mistake with another “simple” solution. He eliminated Leap Day for centennial years, like 1900 and 1700, not divisible by 400.

For Durango resident Ted Holteen, who is 52 – turning 13, his worst Leap Day birthday was when he turned 8.

“That’s when my mom told me I was only 2. I just didn’t understand it yet having no grasp of astronomy,” he said.

Other calendars have leap months, like the Chinese and Jewish calendars. The second Aztec calendar was based on a 365-day solar cycle. It didn’t use a leap day, but it did have about five nemontemi, or nameless days, at the end of the year.

The Solar Hijri calendar, used in Iran and Afghanistan, is one of the world’s most accurate calendar systems. It uses equinoxes, not mathematical rules, to determine the distribution of leap years.

Leap Day birthdays haven’t been so bad, Anderson said, although he was “cheated” out of 54 birthdays. One year, his family and friends threw a surprise birthday party when he turned 10.

“It was at times a bit of a problem,” Anderson said, mainly because early computer calendars did not include Feb. 29. That caused a slight problem when he went to get his driver’s license renewed.

“I told them just put down the 28 because I really needed my driver’s license,” he said.

Holteen said his birthday has always been a good conversation starter.

Because of a shift change and a recording error the night of his birth, Holteen spent most of his life with two birth certificates. One nurse said he was born before midnight on Feb. 28; the other, around 4 a.m. Feb. 29, his true birthday.

“For years, my driver’s license said Feb. 28, and my passport said Feb. 29,” he said.

His attitude toward the day?

“I guess someone had to be born on it,” he said.

smullane@durangoherald.com