millennium bug

from the GNU version of the Collaborative International Dictionary of English.

• noun (Computers) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing A design flaw in computer systems which represented the year with 2 digits (ex. 1984 = 84), which could cause date calculations to fail after 1999.