"Y2K" redirects here. For other uses, see Y2K (disambiguation).

The (French) sign reads 3 January 1900 instead of 3 January 2000

The Year 2000 problem (also known as the Y2K problem, the Millennium bug, the Y2K bug, or simply Y2K) was a 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 1997, The British Standards Institute (BSI) developed a standard (DISC PD2000-1) which identified two major problems that existed in many computer programs.^{[citation needed]}

Firstly, 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, long-working systems would break down when the "...97, 98, 99, 00..." ascending numbering assumption suddenly became invalid.

Secondly, some programmers had misundertood the rule that although years that are evenly divisible by 100 are not leap years, if they are divisible by 400 then they are. Thus the year 2000 was a leap year.

Companies and organizations worldwide checked, fixed, and upgraded their computer systems.

The number of computer failures that occurred when the clocks rolled over into 2000 in spite of remedial work is not known; amongst other reasons is the reticence of organisations to report problems.^[1] There is evidence of at least one date-related banking failure due to Y2K.^[1] There were plenty of other Y2K problems, and that none of the glitches caused major incidents is seen by some, such as the Director of the UN-backed International Y2K Co-operation Centre and the head of the UK's Taskforce 2000, as vindication of the Y2K preparation.^[1] However, some questioned whether the relative absence of computer failures was the result of the preparation undertaken or whether the significance of the problem had been overstated.^[1][2]