Two planes hijacked by Al Qaeda pierced the north and south towers of the World Trade Center. A third slammed into the Pentagon in Arlington, Va. A fourth crashed in an open field outside Shanksville, Pa. All in less than 90 minutes. What, exactly, do you remember? What stories do you tell when a casual conversation morphs into a therapy session? What stories do you keep to yourself? And what instantly transports you back to that deceptively sunny Tuesday morning? In a study of more than 3,000 people, what distinguished the memories of Sept. 11, when compared with ordinary autobiographical memories, was the extreme confidence that people had developed in their altered remembrances. Dan Barry, a longtime Times reporter, remembered “the acrid smell of loss drifting uptown through the newsroom’s open windows. The landfill. The funerals.” Today, he shares an essay about the effects of time on those memories.