Rather than waste characters by entering four-digit dates, programmers entered dates using two digits. To find the number of years between 1991 and 1999, computers subtract 91 from 99. But in two years that will be impossible. Instead of reading 00 as 2000, most computers will interpret it as 1900 and arrive at an incorrect answer, -91, causing everything from bookkeeping errors to system failures.
Companies have several options.
They can buy new computers. They can buy or create new programs to run on their old computers. Almost certainly not. Many states have not assessed the problem yet, much less started repairs. And one study found that 40 percent of U.S. businesses have not started Year 2000 projects.