Millennium bug. Computer artwork representing the millennium bug" by a beetle patterned like a circuit board on a computer screen showing the date 2000. The millennium bug theory is that computers may not understand the change of digits from "99" to "00" at the turn of the century. This is due to the fact that computers only use the last two digits of each year on their internal calendars. There is some debate about the effect this will have. Some say the computers will "think" it is 1900, or even crash, and lose all information."