Source : Free On-Line Dictionary of Computing

Year 2000
     
         (Y2K, or "millennium bug") A common name for all
        the difficulties the turn of the century, or dates in general,
        bring to computer users.
     
        Back in the 1970s and 1980s, the turn of the century looked so
        remote and memory/disk was so expensive that most programs
        stored only the last two digits of the year.  These produce
        surprising results when dealing with dates after 1999.  They
        may believe that 1 January 2000 is before 31 December 1999
        (00<99), they may miscalculate the day of week, etc.  Some
        programs used the year 99 as a special marker; there are
        rumours that some car insurance policies were cancelled
        because a year of 99 was used to mark deleted records.
     
        Complete testing of date-dependent code is virtually
        impossible, especially where the system under test relies on
        other systems such as customers' or suppliers' computers.
        Despite this, the predicted "millennium meltdown" never
        occurred.  Various fixes and work-arounds were successfully
        applied, e.g. {time shifting}.
     
        And yes, the year 2000 was a leap year (multiples of 100
        aren't leap years unless they're also multiples of 400).
     
        {PPR Corp Y2K FAQ
        (http://www.pprcorp.com/y2k/y2kfaq_j97.html)}.
     
        (2003-08-15)