In computer programs, 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.