High Quality Content by WIKIPEDIA articles! The Year 2000 problem (also known as the Y2K problem, the millennium bug, the Y2K bug, or simply Y2K) was a notable problem for both digital (computer-related) and non-digital documentation and data storage situations which resulted from the practice of abbreviating a four-digit year to two digits. In computer program design, the practice of representing the year with two digits becomes problematic with logical error(s) arising upon "rollover" from x99 to x00. This has caused some date-related processing to operate incorrectly for dates and times on and after January 1, 2000 and on other critical dates which were billed "event horizons". Without corrective action, it was suggested that long-working systems would break down when the "...97, 98, 99, 00..." ascending numbering assumption suddenly became invalid. Companies and organizations worldwide checked, fixed, and upgraded their computer systems.