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FOXBORO - Meet Foxboro's most grown-up children.

John Balsavich of Spruce Street will celebrate his 12th birthday today. Yet he is past chairman of the town's advisory committee and an MIT-educated engineer with a senior position in a biomedical company.

Linda Hobbs works in retail, and has reared three children in her Garfield Street home.

"Officially, I will be 14 (today)," Hobbs said.

And Gail Brown, an at-home mom, has heard a few birthday zingers from family members. Born Feb. 29, 1976 - on Leap Year Day - Brown will see her 8th birthday tomorrow, making her just one birthday ahead of her daughter, Britney, who turned 7 last Sunday.

"Soon you'll be older than your mother," Brown said one of her brothers teased Britney during the child's party last weekend.

Feb. 29 comes but once every four years. Balsavich explains why. It takes the Earth about 365-1/4 days to orbit the sun. Every fourth year, a day is added to the calendar to account for the surplus time.

Otherwise, he said, over many years the calendar would slip further and further out of synch with the seasons.

"Summer would be winter," Balsavich said.

Leap Year Day is the subject of a few Web sites, such as Leapzine (www.leapyearday.com).

"February 29 is probably THE most important day on the calendar because of the position it holds and the duty it performs," the site says. "It is there to maintain balance between the calendar and the spinning earth."

The site urges people to think of Feb. 29 as an extra day, and make good use of it.

In the late 1800s and early 1900s, Leap Year Balls "provided women the opportunity to propose" to the men of their choice.

Balsavich, Hobbs and Brown each have their own way of dealing with the missing birthdays. Balsavich and his wife, Kathy, more or less ignore adult birthdays, but do celebrate the births of their children, Jay, 21, and Hannah, 18, a senior at Foxboro High School.

Hobbs celebrates her birthdays on Feb. 28 "but the real ones are really special," with more parties, dinners and presents. She'll have her party tonight in Hyannis with some girlfriends, kicking off a weekend on the Cape among the friends.

Charles and Linda Hobbs have lived in Foxboro for 33 years, and have two daughters and a son.

"Even though its not a Leap Year, I'm sending you this birthday card," one of her daughters wrote her last year.

Brown, an at-home mother, remains a Leap Day purist.

Apart from the ice cream cake her husband, Matt Brown, brings home each year on Feb. 28, she celebrates her own birthday only when Feb. 29 returns for real.

She doesn't feel deprived.

One year, as a child growing up in Roslindale, friends and family gave her six birthday parties: at home, at sleepovers and at the movies.

"It was just everywhere and my brothers were so jealous," said Brown, who has four brothers, all younger than her.

Younger than her legal age, that is.

Today, Brown will turn 32, Balsavich 48, Hobbs 56.

But don't try to steal their youth by counting the years gone by.

"We joke in my family that both of my kids are older than I am," Balsavich said. "Right now I'm younger than the dog in our house."

The family's yellow Labrador retriever, Bailey, is 13.

Now in "dog years," that makes Bailey...

Oh, never mind.

You'd need to be an M.I.T. grad to orbit with the mathematical big dogs.

So let it be a simple happy birthday to Gail, 8, to John, 12, and to Linda, 14.