BEIJING — When President Hu Jintao said in a pomp-filled ceremony in the Great Hall of the People on July 1, “The Communist Party of China was founded 90 years ago today,” and, “From then on, the Chinese people embarked on the bright road of striving for independence and liberation and the glorious pursuit of prosperity and strength,” he was wrong.

About the date, anyway.

The party’s true founding date is July 23, 1921, according to official documents. An autobiographical account by one of the party’s founders and, remarkably, memories and news reports of a murder in a Shanghai hotel corroborate that, providing important clues for historians keen to establish the truth.

Ultimately, the strange story of the wrong founding date of the world’s biggest political party is the result of Mao Zedong’s mistaken memory and the party’s disregard for truth, historians say.

While the true date is now recorded in some Chinese history books, to this day most Chinese are unaware of the error and are astonished when they hear of it. For months, people had been exposed to intense propaganda about the July 1 anniversary from the news media, workplaces and slogans festooning roads.