The Year 2000 problem (also known as the Y2K problem, the millennium bug, the Y2K Bug, or simply Y2K) was the result of a practice in early computer program esign that caused some date-related processing to operate incorrectly for dates and times on and after between December 31, 1999 and January 1, 2000 and on other critical dates which were billed "event horizons". This fear was fueled by the attendant press coverage and other media speculation, as well as corporate and government reports. People recognized that long-working systems could break down when the "...97, 98, 99..." ascending numbering assumption suddenly became invalid. Companies and organizations world-wide checked and upgraded their computer systems.