And a number of companies and military organisations are closing e-mail and Web sites to thwart hackers or computer viruses in the early hours of the millennium, when anomalies might be wrongly interpreted as a "year 2000" software problem.
In Britain, retailers wrestled with a software problem in terminals made by Racal. It meant any transaction in the four days before 2000 would not initially be allowed, leaving the machines' users scrabbling for paper slips to record the sale.
Credit and debit cards from all issuers, including Visa and American Express, were affected. A spokeswoman for HSBC Bank, which issued most of the terminals, said: "The fault will be completely cleared by January 1." Only 20,000 of the 600,000 swipe terminals in Britain are affected, said the Association of Payment Clearing Services, which oversees inter- bank money transfers. Bankers insisted however that the problem was not relevant to internal banking systems.
A spokesman for Racal said: "This has shaken us badly. We did test for dates close to the end of the century but this problem would not have shown up immediately. The machine has to be running in that state for a few minutes. By that time the tester would have moved on to the next test."
Fears of hacker attacks led companies including Glaxo Wellcome, the pharmaceuticals giant, and Vauxhall, Volkswagen and Ford to block external e-mail over the weekend.
The US military is also closing a number of its public sites. "If you're not up on the 31st, there is nothing they [hackers] can do about it," said the US Air Force spokesman. "There is no specific threat; it's simply heightened security."