The date-related computer problem widely feared as 1999 changed to 2000 was the Y2K problem.
Y2K, short for “Year 2000,” arose because many computer systems stored years as two digits, such as 99, rather than four digits, such as 1999. When the calendar reached January 1, 2000, software might interpret “00” as 1900 instead of 2000. That could affect calculations involving dates, records, schedules, finance, and embedded control systems.
Governments, companies, and software engineers spent the 1990s auditing and repairing systems. Midnight celebrations on December 31, 1999, therefore carried an unusual technological anxiety alongside the usual festivities. The most catastrophic predictions did not occur, partly because extensive preparation had addressed many vulnerable systems.
Y2K was not caused by the Gregorian calendar itself, and it was not the same as the later Year 2038 problem, which concerns signed 32-bit Unix time. The phrase “millennium bug” was a popular label for the issue.