Today's date has haunted the world for a decade and a half. The digits 9/11 are universal shorthand for the horror of that morning and all that has happened as a result. Terrorism took on a new dimension in the hands of people who would fly aeroplanes into buildings and strap explosives to their bodies.
The hijackings and hostage-taking typical of 20th century terrorism was bad enough. But while those fanatics were prepared to die for their cause, they hoped not to. Terrorism in the 21st century is calculated suicide.
It is hard to protect any society from an anonymous individual or even a team of them, who plan to kill themselves in an act of mass murder.
These acts have become steadily more common in the 15 years since 9/11. The "war on terror" launched by US President George W Bush later that day, may have detected and prevented more such attacks in the US but it took a wrong and disastrous turn when he had flushed al-Qaeda and the Taliban from Afghanistan and decided to invade Iraq.
Al-Qaeda, Taliban, more words few had heard before 9/ll. Now hardly a day passes when militants deriving deadly inspiration from one of the world's great religions is not in the news. The rest of the world hardly gave Islam a thought before 9/11. Now we are well acquainted with Sunni, Shia, jihad and the garments that shroud women.
These are among the last subjects Western society expected to be worrying about in the 21st century, and still among the last subjects most want to be thinking about. The brand of Islam confronting us today belongs in medieval history.
But thanks to 21st century technology, seventh century jihadism has not only had a revival in Islamic countries, its barbarism is infecting sick or alienated minds in all countries.
After 15 years we seem no closer to understanding, let alone defeating, the threat that announced itself in New York that morning. But New York has a new tower. Terror will not define our times.