The Year 2000 problem, also known as the Y2K problem, the Millennium bug, the Y2K bug, or Y2K, was a computer bug related to the formatting and storage of calendar data. 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.