An error in the coding of certain computer programs which store the year component of the date as two digits, assuming that the first two digits are 19, rather than as a complete number of four digits; when such programs are used after January 1, 2000, the date may be misinterpreted, causing serious errors or total failure of the program; -- called also year 2000 bug, year 2000 problem and Y2K bug.

The problem that arises when computers cannot recognize the date changeover from the last two digits of 1999 to the year 2000.

In the beginning, computers were huge, and their memory capability was small. Today the reverse is true. In the beginning the space on a timer microchip for holding such mundane routines as the date, was at a premium, so short cuts were used. Instead of a date being given eight figures such as 01011998, - First of January, 1998, - it was saved as 010198. This was fine until it occurred to the world to pass the millennium date. Any program that compares dates under the old program code will be confused because it will assume that 010101 comes before 010199, not realising that we are into the next millennium. Computer programs are basically stupid, and when they get confused, they go into an old fashioned sulk, and either produce nothing while they contemplate the problem which causes a hang-up, or produce unpredictable results just like a cantankerous child.

the inability to spell the word millennium correctly.