Y2K

The Year 2000 problem is also known as the Y2K problem, the Millennium bug, the Y2K bug, or Y2K. Problems arose because programmers represented the four-digit year with only the final two digits. This made the year 2000 indistinguishable from 1900. The assumption that a twentieth-century date was always understood caused various errors, such as the incorrect display of dates, and the inaccurate ordering of automated dated records or real-time events.

In 1997, the British Standards Institute (BSI) developed a standard, DISC PD2000-1, which defines "Year 2000 Conformity requirements" as four rules: 1) No valid date will cause any interruption in operations. 2) Calculation of durations between, or the sequence of, pairs of dates will be correct whether any dates are in different centuries. 3) In all interfaces and in all storage, the century must be unambiguous, either specified, or calculable by algorithm. 4) Year 2000 must be recognised as a leap year.

It identifies two problems that may exist in many computer programs. First, the practice of representing the year with two digits became problematic with logical error(s) arising upon "rollover" from x99 to x00. This had caused some date-related processing to operate incorrectly for dates and times on and after 1 January 2000, and on other critical dates which were billed "event horizons". Without corrective action, long-working systems would break down when the "... 97, 98, 99, 00 ..." ascending numbering assumption suddenly became invalid.

Secondly, some programmers had misunderstood the Gregorian calendar rule that determines whether years that are exactly divisible by 100 are not leap years, and assumed the year 2000 would not be a leap year. Years divisible by 100 are not leap years, except for years that are divisible by 400. Thus the year 2000 was a leap year.

Companies and organisations worldwide checked, fixed, and upgraded their computer systems.

The number of computer failures that occurred when the clocks rolled over into 2000 in spite of remedial work is not known; among other reasons is the reluctance of organisations to report problems.

Y2K is a numeronym and was the common abbreviation for the year 2000 software problem. The abbreviation combines the letter Y for "year", and k for the SI unit prefix kilo meaning 1000; hence, 2K signifies 2000. It was also named the "Millennium Bug" because it was associated with the popular (rather than literal) roll-over of the millennium, even though the problem could have occurred at the end of any ordinary century.

...
Wikipedia