The Year 2000 problem, also known as the Y2K problem, the Millennium bug, the Y2K bug, or Y2K, is a class of computer bugs related to the formatting and storage of calendar data for dates beginning in the year 2000.
Problems were anticipated, and arose, because twentieth-century software often represented the four-digit year with only the final two digits—making the year 2000 indistinguishable from 1900.
The assumption of a twentieth-century date in such programs caused various errors, such as the incorrect display of dates and the inaccurate ordering of automated dated records or real-time events.
Proper Prior Planning Prevents Pi** Poor Performance.
A huge amount of time, money & effort was put in to checking, updating & where necessary replacing hardware & software - prior to the Y2k rollover: when 1999 became 2000.