Three-thousand six hundred fifty-three days have now passed. At 8:46 a.m. — the time when the first plane slammed into 1 World Trade Center — 87,672 hours will have gone by. Another 5,260,320 minutes. Another 315,619,200 seconds.

Once more the families will gather at ground zero, where 2,749 died, and in Washington and in Pennsylvania to pay tribute to the 224 who died there.

Once more, there will be an outpouring of grief. Once more, there will be the sound of bells tolling in mourning. Once more, there will be speeches. Once more, the names will be recited.

But the 10th anniversary will dawn on a city and a nation that has changed immutably, with continuing wars in Afghanistan and Iraq and persistent security worries at home. And no longer is ground zero a scarred reminder of what was, but a symbol of resurgence, with the September 11 National Memorial about to open and four new towers in various stages of construction.

New York got a jittery reminder about security last week with renewed police checks of cars and tracks. The inspections — and the quarter-mile-long traffic jams they created in places — followed unconfirmed reports that Al Qaeda operatives were had been planning an attack to coincide with the anniversary.

One measure of how Sept. 11 changed everything, though, was how little grumbling there was as motorists waited to crawl through police checkpoints. Sept. 11 redefined the bridges and tunnels beyond those checkpoints as something that generations of commuters had never imagined: Potential targets.

Sept. 11 redefined so much more.

Sept. 11 put New York, a city that had not faced combat in more than 200 years, on the front lines in a global war on terrorism. Sept. 11 made slogans created by Madison Avenue like “If you see something, say something” as widespread as “Loose lips sink ships” once was.

Sept. 11 brought color-coded threat levels (though the Department of Homeland Security, itself a post-Sept. 11 creation, phased them out several months ago). Still travelers worry: Is it safe to fly? Since Sept. 11, airline passengers have had to pull off their shoes and empty their pockets, and they felt embarrassed when they forgot they had a too-big bottle of shampoo or mouthwash in their carry-on.

And still there were episodes when terrorists on international flights tried to set off plastic explosives hidden in their shoes or sewn into their underwear.

Is it safe to open the mail? A week after the Sept. 11 attacks, letters containing anthrax killed five people and infected 17 others. It took the F.B.I. five years to conclude that an Army microbiologist had been responsible. In the confusion at first, people hoarded antibiotics, and officials briefly grounded crop-dusting airplanes.

But this anniversary played out against a different backdrop than the first anniversary, in 2002, or the fifth, in 2006. For the first time, Osama bin Laden was dead. “We’ve taken the fight to Al Qaeda like never before,” President Obama declared Saturday in his weekly radio address.

For the first time, too, there was tangible progress toward fulfilling the promise to rebuild — a promise made in the aftermath of the attacks but delayed by squabbling over architects, plans and finances. Buildings are rising between Church and West Streets in Lower Manhattan, and the National September 11 Memorial will open to the public on Monday. Relatives of those who died at the World Trade Center will get a first look on Sunday.

If they were to measure it, they would see that the memorial covers about half of the 16-acre World Trade Center site. They will see that the names of the dead have been inscribed on the walls of two reflecting pools that now fill the footprints of the old towers. A museum is to open nearby next year.

This time, there will be other reminders. The U.S.S. New York, commissioned in 2009 and made with seven and a half tons of steel from the twin towers, spent the weekend at anchor in the Hudson River. On Sunday morning it was to cruise to Lower Manhattan, stopping within sight of the new tower at the trade center site.

Other ceremonies and services are planned. The New York City Fire Museum will honor the 343 firefighters who died with the dedication of the bunker coat and helmet that a Fire Department chaplain, Mychal Judge, was wearing on Sept. 11 when he perished. Marble Collegiate Church in Manhattan will have a “trialogue,” a three-way discussion with Shamsi Ali, the imam of the Islamic Cultural Center of New York; Rabbi Michael S. Friedman, the associate rabbi of Central Synagogue in Manhattan; and Dr. Michael B. Brown, the church’s senior minister.

At night, an interfaith ceremony on the south side of Pier 40, a park at the west end of Houston Street, will be led by the Rev. Alfonso Wyatt, the vice president of the Fund for the City of New York.

The ceremony at ground zero will bring together the officials who were in office 10 years ago — President George W. Bush, Gov. George E. Pataki of New York, Gov. Donald T. DeFrancesco of New Jersey and Mayor Rudolph W. Giuliani — with their successors: President Obama, Gov. Andrew M. Cuomo of New York, Gov. Chris Christie of New Jersey and Mayor Michael R. Bloomberg.

As at past observances, there will be music. The cellist Yo-Yo Ma, who performed at the one-year anniversary ceremony, is to perform the Sarabande from Bach’s Suite for Cello No. 1. James Taylor is to perform “Close Your Eyes” and Paul Simon is to perform “Bridge Over Troubled Water.”

The ceremony is to pause six times: twice to remember the planes that hit the towers, twice to remember when the towers collapsed, once for the attack on the Pentagon and once the plane that went down in a field in Pennsylvania. The first moment of silence is scheduled for 8:46 a.m., when American Airlines Flight 11 sliced into 1 World Trade Center — the north tower —17 minutes before United Airlines Flight 175 hit the south tower.

And still what happened on that shining morning seems as impossible as it did in those first few minutes, when one friend called another and said something like, “Go turn on the television. A plane has crashed into the World Trade Center.”

Or when, in the seconds before the picture came on, an anchor was heard saying something like, “Wait. These are live pictures, not the tape? So that was a different plane, and it hit the other one?”

Like the day when John F. Kennedy was assassinated in 1963, or the day when the space shuttle Challenger exploded in 1986 or the day when Pearl Harbor was attacked in 1941, Sept. 11 was one of those days that divided things into “before” and “after.”

New Yorkers still talk about what a bright morning that was, after a thunder-and-lightning show in the sky the night before. They talk about how late-summer days are forever different. They talk about how the foreboding that has replaced the promise in the pink of the sunrise and so much joy in the deep blue of the midmorning sky.

And they talk about what the World Trade Center was, a city-within-the-city that dominated the skyline. Below 14th Street it was a direction-finder as sure as the “N” on any compass. It had been bombed in 1993. The damage had been repaired, but the two buildings remained a target for Al Qaeda.