Year 2000 problem
The pervasive problem that many applications are designed to handle only 20th-century dates -- dates that begin with "19". For example, most programs represent dates in the form MM-DD-YY, so the date 10-5-96 is October 5, 1996. But what about the date 10-5-05. Is that 1905 or 2005? There is no way to distinguish between these two dates.
This problem affects a vast amount of software, particularly accounting and database systems. The U.S. Social Security Administration, for example, has estimated that it will need to review about 50 million lines of code to correct this problem in its own system.
The Year 2000 problem is sometimes referred to as the Millennium bug or Y2K problem.