President Biden for once beat the clock.

The 46th president, known for his chronic tardiness, finished reciting his oath of office at 11:48 a.m. Wednesday, with his hand on a five-inch-thick Bible that has been in his family for 127 years.

The ceremonies proceeding ahead of schedule had some wondering if Biden’s swearing-in ahead of noon meant he assumed the presidency 12 minutes early. But ultimately it’s the 20th Amendment — not the inaugural itinerary — that determines when a new presidential term begins, no matter if the oath takes place ahead of time, according to constitutional experts.

The 20th Amendment states that the “terms of the President and Vice President shall end at noon on the 20th day of January.”

George Washington University law professor Jonathan Turley said it’s not taking the oath that results in an incoming president taking office.

“The oath is required but it is not the act that makes Biden the next president,” Turley told The Washington Post in an email. Turley noted that no matter how early the president-elect swears the oath of office, the previous president — barring a last-minute resignation — remains the officeholder until noon.

“So when Sen. Amy Klobuchar said that she would be the first to call him ‘President Biden,’ she may have been literally but not constitutionally correct,” Turley said.

Robert Chesney, a law professor at the University of Texas at Austin, pointed to Article II, Section 1 of the Constitution, which states that the oath must be taken “before” the president takes office.

“Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation,” the clause reads.

“The prior president’s authority expires no matter what at noon on the 20th,” Chesney said in an email, adding, “it would actually be a bit weird if there were some sort of rule requiring that moment be passed before the new president could take the oath effectively, since this would create at least a few moments if not several minutes in which neither one would be president.”

Wednesday’s slight upending of the highly choreographed Inauguration Day schedule is not unheard of in U.S. history, though some of the details are too important to compromise: In 2009, President Barack Obama was sworn in for a second time Jan. 21 after he and Chief Justice John G. Roberts Jr. both seemingly stumbled over the 35-word oath whose exact language is specified by the Constitution.

White House lawyers, acting out of an “abundance of caution,” opted to have Roberts re-administer the oath the next day.

“We decided it was so much fun” the first time, Obama joked of the do-over.

At a small, private ceremony in the White House Map Room, both Obama and Roberts nailed the words.