Will the millennium arrive in darkness and chaos as billions of lines of computer code containing two-digit year dates shut down hospitals, banks, police and fire departments, airlines, utilities, the Pentagon, and the White House? The nightmare scenarios are only too possible, ROBERT SAM ANSON discovers as he traces the birth of the Y2K "bug," the folly, greed, and denial that have muffled two decades of warnings from technology experts, and the ominous results of Y2K tests that lay bare the dimensions of ticking global time bomb