It's hard to pinpoint the exact moment when the paranoia first began to creep in. Sometime during the late 1990s, consumers noticed that their credit cards with expiration dates in the year 2000 were being declined by merchants. Shortly thereafter, people began stocking up on shelf-stable food and water, potentially condemning themselves to months of all-SPAM diets. A number of concerned citizens outside of Toronto, Canada, flocked to the Ark Two Survival Community, a nuclear fallout shelter-turned-bunker comprised of dozens of decommissioned school buses buried several feet below the Earth and protected by a layer of reinforced concrete.

In the months leading into New Year's Day 2000, millions of people steeled themselves for a worst-case scenario of computers succumbing to a programming glitch that would render them useless. Banking institutions might collapse; power grids could shut down. Anarchy would take over. The media had the perfect shorthand for the potential catastrophe: Y2K, for Year 2000. The term was used exhaustively in their coverage of a situation some believed had the potential to become one of the worst man-made disasters in history—if not the collapse of modern civilization as we knew it.

In the end, it was neither. But that doesn't mean it didn't have some far-reaching consequences.

Michael Smith, Getty Images

The anticipatory anxiety of Y2K was rooted in the programs that had been written for the ginormous computers of the late 1960s. In an effort to conserve memory and speed up software, programmers truncated the date system to use two digits for the year instead of four. When the calendar was set to roll over to the year 2000, the belief was that "00" would be a proverbial wrench in the system, with computers unable to decipher 2000 from 1900. Their calculations would be thrown. Using "98" for 1998 was a positive value; using "00" would result in negative equations. How computers would react was based mostly on theories.

That ambiguity was quickly seized upon by two factions: third-party software consultants and doomsday preppers. For the former, rewriting code became a cottage industry, with corporations large and small racing to revise antiquated systems and spending significant amounts of money and manpower in doing so. General Motors estimated the cost of upgrading their systems would be about $626 million. The federal government, which began preparing for possible doom in 1995, ended up with an $8.4 billion bill.

Some of that cost was eaten up by soliciting analyses of the potential problems. The U.S. Department of Energy commissioned a study looking at the potential for problems with the nation's energy supply if computers went haywire. The North American Electric Reliability Council thought the risks were manageable, but cautioned that a single outage could have a domino effect on connected power grids.

As a result, many newspaper stories were a mixture of practical thinking with a disclaimer: More than likely nothing will happen … but if something does happen, we're all screwed.

"Figuring out how seriously to take the Y2K problem is a problem in itself," wrote Leslie Nicholson in the January 17, 1999 edition of the Philadelphia Inquirer. "There is simply no precedent."

Pending economic and societal collapse fueled the second pop-up industry: survivalist suppliers. As people stocked up on canned goods, bottled water, flashlights, and generators, miniature societies like Ark Two began to spring up.

While the panic surrounding Y2K was dismissed by some as unwarranted, there was always fuel to add to the fire. The United States and Russia convened to monitor ballistic missile activity in the event a glitch inadvertently launched a devastating weapon. People were warned checks might bounce and banking institutions could freeze. The Federal Reserve printed $70 billion in cash in case people began hoarding currency. Even the Red Cross chimed in, advising Americans to stock up on supplies. Y2K was being treated like a moderate-category storm.

Adding to the concern was the fact that credible sources were sounding alarms. Edward E. Yardeni, then-chief economist at Deutsche Morgan Grenfell/C.J. Lawrence, predicted that there was a 60 percent chance of a major worldwide recession.

As New Year's Eve 2000 approached, it became clear that Y2K had evolved beyond a software hiccup. Outside of war and natural disasters, it represented one of the few times society seemed poised for a dystopian future. People watched their televisions as clocks hovered close to midnight, waiting to see if their lights would flicker or their landline phones would continue to ring.

Of course, nothing happened. So many resources had been extended toward the problem that the majority of software-reliant businesses and infrastructures were prepared. There were no power outages, no looting, and no hazards. The only notable event of January 1, 2000 was the reporting of the resignation of Boris Yeltsin and the arrival of Vladimir Putin as Russia's new president.

With the benefit of hindsight, pundits would later observe that much of the Y2K concern was an expression of a more deeply rooted fear of technology. Subconsciously, we may have been primed to recoil at the thought of computers dominating our society to the extent that their failure could have catastrophic consequences.

All told, it's estimated that approximately $100 billion was spent making upgrades to offset any potential issues. To put that into context: South Florida spent $15.5 billion rebuilding after the mass destruction caused by Hurricane Andrew in 1992.

Was it all worth it? Experts seem to think so, citing the expedited upgrades of old software and hardware in federal and corporate environments.

That may be some small comfort to Japan, which could be facing its own version of Y2K in April 2019. That's when Emperor Akihito is expected to abdicate the throne to his son, Naruhito, the first such transition since the dawn of the information age. (Akihito has been in power since January 1989, following the death of his father.) That's significant because the Japanese calendar counts up from the coronation of a new emperor and uses the name of each emperor's era. Akihito's is known as the Heisei era. Naruhito's is not yet named, which means that things could get tricky as the change in leadership—and the need for a calendar update—comes closer.

It's hard to predict what the extent of the country's problems will be as Akihito steps down. If history is any guide, though, it's likely to mean a lot of software upgrades, and possibly some SPAM.