The computer problem feared to disrupt systems at the turn of the millennium was called Y2K, or the Year 2000 problem.
Many older computer programs stored years with only two digits, such as “99” for 1999. Some systems could interpret “00” as 1900 rather than 2000, potentially causing errors in calculations, sorting, dates, and automated processes.
Governments, businesses, utilities, and software developers spent years auditing and updating systems before December 31, 1999. Midnight passed with fewer major failures than many predictions had suggested, although some localized problems and incorrect date displays occurred.
Y2K was not itself a New Year’s Eve celebration; it was a technology risk associated with the date change. The widespread preparation is one reason the transition is often discussed alongside millennium celebrations.