— The first day of January isn’t celebrated as New Year’s Day in Afghanistan. But since the American invasion, it’s become a new kind of holiday — a de facto birthday for thousands of Afghans who don’t know when they were born.

During protracted wars in the 1980s and ’90s, the government didn’t have a system in place to register births. Because identification cards and driver’s licenses weren’t standard in this impoverished nation, families saw no reason to record the exact dates. Government paperwork asked only for an approximate birthday on the Islamic calendar.

But when the United States and its NATO allies arrived, they brought with them a flurry of job opportunities, visa applications and Web sites that all required a specific birthday on the Roman calendar.

“Those of us who don’t know when we were born selected January 1st,” said a U.S. Army interpreter named Tariq, who first wrote the date on his job application with the military and would repeat it when he applied for a visa, and whenever anyone asked. “It was very easy to remember.”

Like many Afghans, Tariq, who requested that his last name not be used to avoid Taliban threats, has only a vague sense of his birthday, which occurred during the country's collapse into civil war in the early 1990s.

The question of birthdays has arisen with even greater frequency as Internet access has become more widespread, with 3G networks advertised in the country’s major cities. Urban Afghans were quick to create accounts on Facebook, Twitter and Gmail, all of which ask for the registrant’s date of birth.

In the digital age, the collective birthday has become something of an inside joke here, as young Afghans send each other messages to celebrate.

“Happy birthday to 30 friends . . . whose birthdays are tomorrow on the first of January,” Barat Ali Batoor, an Afghan refugee in Australia, wrote on Facebook.

“In two days, it’s every Afghan’s birthday,” Mohammad Hassanzai, an Afghan living in London, tweeted Monday.

“I have been using the 1st of January for every online registration and social network site,” said Nazer Hussain, 23, a recent university graduate who rattled off a list of Web sites he signed up for using his fake birthday. “In the past, people weren’t well-educated enough to keep a record of birthdays.”

Some worry that the lack of official birth registration — a problem that persists today, particularly in rural parts of the country — could have serious implications.

“Birth registration is instrumental in safeguarding other human rights because it provides the official ‘proof’ of a child’s existence,” said a 2007 U.N. report on the topic. It listed Afghanistan as one of the 10 countries with the largest numbers of unregistered children.

The birth records are useful in reuniting families after conflicts or natural disasters, as well as in helping children apply for refugee status. And in the absence of a census, institutionalized birth registration can offer an estimate of the country’s population.

“In Afghanistan, even though national legislation requires registration of children at birth, 23 years of conflict decimated both the administrative mechanisms and the social institutions that support them,” the U.N. report said.

Afghanistan isn’t the only war-torn nation whose citizens have chosen Jan. 1 as a makeshift birthday. In Vietnam, Somalia and Sudan, many birth dates weren’t recorded during years of unrest and institutional upheaval. When residents applied for visas or refu­gee status, thousands chose the first of the year — or, in some cases, the U.S. State Department chose it for them. The department has bestowed that birthday upon more than 200,000 refugees since the Vietnam War, according to several estimates.

“These approximated birth dates allow the government to administer benefits and track and control immigration flow, but they lack both certainty and accuracy,” Ross Pearson wrote in December in the Minnesota Law Review.

Afghans didn’t wait to be assigned official birthdays. As U.S. immigration lawyers have accepted hundreds of visa applications, they noticed that many of their clients had already filled in their date of birth as Jan. 1. Many Afghans who are also unsure which year they were born offered their best guess on that line.

Most say they chose Jan. 1 because it was the easiest date to remember. But young Afghans in particular have coalesced around it — celebrating a mass birthday that is also an implicit acknowledgment of their country’s troubles.

The country’s famous actors, such as Basir Mujahid; its athletes, such as cricket player Hasti Gul Abid; and its politicians, such as Mohammad Daud Daud, the former police chief of northern Afghanistan, all publicly celebrate their birthdays on Jan. 1.

Many Afghans, particularly the young, digitally savvy generation, remain curious about their true birthdays. But their parents don’t offer much clarity, or at least enough to warrant a change.

“I’m not sure,” Hussain said about his real birthday. “I think it was sometime in the spring.”

Special correspondent Mohammad Sharif contributed to this report.