a bug that can cause computers or software to misinterpret the first two digits of the year 2000 as 19, due to the coding of dates using only the last two digits of the year.
Origin of millennium bug
Also called Y2K problem, Y2K bug.
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British Dictionary definitions for millennium bug
computing any software problem arising from the change in date at the start of the 21st century
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012