With the millennium-bug or 00-bug is meant that many computers nowadays in use cannot distinguish between the year 1900 or 2000 or 2100 etc. Because they only calculate dates with two numbers. With the datashift from 31.12.1999 to 01.01.2000 therefore catastrophic events are predicted, from miscalculations to system-crashes and it is nearly impossible to fix the bug, because all the sourcecodes of the old software and even some hardware has to be reviewed and changed.