This is due to the common practice used by programmers, until recently, of storing the year portion of a date as a two digit number. Only the last two digits of the year are represented. Most people can accept this format as unambiguous, assuming that the leading digits for the year are 19.
Computers find it more difficult to make that leap.
What happens at this point depends in large part on the computer program. Couple this with the role of computers in almost all facets of business and the fact that many of the programs still running on computers today were written years ago without regard to the eventual millennium rollover and you can see that the world is faced with a big challenge in dealing with dates beyond January 1, 2000. A monumental undertaking that is currently diverting the attention of hundreds of thousands of programmers throughout the world. But that's not all. The practice of representing year dates as two digit numbers was so commonplace in programming that it made its way into many of the computerized control systems for appliances, telecommunications systems and utilities.