The 69-year-old Republican front-runner made the blunder while campaigning on the eve of the high-stakes presidential primary in New York by getting the date of the World Trade Center attacks in 2001.
"I wrote this out, and it's very close to my heart," he said at the outset of his remarks on Monday.
"Because I was down there and I watched our police and our firemen down at 7/11, down at the WorldTradeCenter right after it came down. And I saw the greatest people I've ever seen in action," he said at a campaign rally at the FirstNiagaraCenter in downtown Buffalo.
The billionaire businessman did not correct himself, US media reported.
The slip of the tongue could prove embarrassing for Trump as he has repeatedly mentioned the terror attacks while campaigning across New York, his home state, some reports said.
Trump has held up New York city's response to the September 11, 2001 terrorist attacks as a defining "New York value."
That's the phrase used disparagingly by Texas Senator Ted Cruz several months ago, which Trump has tried to turn on his Republican rival, CNN reported.
During his speech, Trump told supporters that "no New Yorker" could vote for his rival Cruz, as he did not represent what the state needs.
He also expressed confidence that he would secure the 1,237 delegates required for him to become the Republican party's presidential nominee, despite claims that he may fall short -- leading to a contested convention.
In the run-up to primary, Trump was widely predicted to win comfortably during the contest in New York, which has 95 Republican delegates up for grabs.
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