As corporate executives in the Americas and Europe prepare for, celebrate, and then recover from New Year's Eve, thousands of bank employees will be working to assure a successful New Year's Day debut for euro, Europe's common currency. Major banks across Europe have set up "war rooms" to manage the initial changeover in 11 of the European Union's 15 member countries. By choice the United Kingdom, Denmark, and Sweden are not taking part in euro's debut. And Greece did not economically qualify. Publication of exchange rates between national currencies and the euro on New Year's Eve will set off a frantic 100-hour effort to convert cash balances, redenominate bond and derivative portfolios, and make final changes to computer systems. Governors of the soon-to-be history European central banks have been told to be within an hour of a secure phone to deal with any emergency. "We have to be ready for the unlikely eventuality that something, somewhere can go wrong," says Wim Duisenberg, president of the new European Central Bank and the person who will have responsibility for monetary policy among the 11 nations in what's informally being dubbed Euroland.