DETROIT, Mich. — Punctuality has never been one of Mayor Bill de Blasio’s strong suits — and he proved it yet again here on Tuesday.

Hizzoner had been scheduled to appear at a pre-debate event in Detroit with other 2020 Democratic hopefuls, which was hosted by Michigan Rep. Brenda Lawrence, but he didn’t show up.

“[De Blasio] committed to me. He said he would be late. And now I just heard he wasn’t coming,” Lawrence told The Post.

“It’s his loss,” she said. “He had a room full of young people to talk to. I wanted to hear his speech — I really did because I’m a former mayor. I tried. I invited. He accepted.”

Lawrence announced de Blasio was coming to a packed ballroom full of young Democrats and Detroit- area constituents, but was left in the lurch when the mayor didn’t show.

“The fact that he’s not here, yeah, we’re bummed out,” said West Bloomfield resident Shirya Beessabathuni.

New York Sen. Kirsten Gillibrand, who was also scheduled to appear, didn’t show up either — but at least gave an excuse, according to Lawrence.

“Family emergency,” the lawmaker said of Gillibrand’s reasoning for not attending. De Blasio, however, gave no explanation.

“It was a bit rude,” Beessabathuni told The Post.

California Sen. Kamala Harris was the only 2020 presidential candidate who managed make an appearance.

“I thought it was very respectful of her,” Beessabathuni said, “and made up for the people who didn’t show up.”

De Blasio’s camp told The Post in a statement on Tuesday night that it was sorry for missing the event but claimed that it had never promised the mayor would be there.

“While we apologize if there was any misunderstanding we never confirmed our attendance,” said spokesperson Olivia Lapeyrolerie. “We also told the organizers we were unable to attend.”

The mayor has been notoriously tardy during his two terms as mayor — and his punctuality problem has also spilled over to his presidential campaign.

He showed up 41 minutes late for a live TV interview earlier this month and later blamed the delay on his alarm clock not being set properly.

“I know we teased Mayor de Blasio at 7:30. He is running late,” PIX11 Morning News co-host Dan Mannarino told viewers. “He was in Chicago last night, so this was an early wake-up.”

The mayor never explained why he kept everyone waiting so long, but a mayoral spokeswoman was able to confirm that his alarm clock was to blame.

The Post famously gave de Blasio an alarm clock — with dual exposed bells and no snooze button — back in 2014 in a vain attempt to get him to his appointments on time.

The move came after Hizzoner sparked outrage by arriving 20 minutes late to an event — and missing the tolling of a bell — marking the 13th anniversary of the American Airlines Flight 587 crash, which killed 260 people on board and five more on the ground in Queens.

He, Harris and Gillibrand will all be part of the Democratic debate on Wednesday, which also includes former Vice President Joe Biden, Washington Gov. Jay Inslee, Rep. Tulsi Gabbard of Hawaii, Colorado Sen. Michael Bennet, New Jersey Sen. Cory Booker, entrepreneur Andrew Yang and Julián Castro, former housing secretary in the Obama administration.

The other candidates — which include Rep. Beto O’Rourke of Texas, Montana Gov. Steve Bullock, author and activist Marianne Williamson, former Rep. John Delaney, former Colorado Gov. John Hickenlooper, Rep. Tim Ryan of Ohio, Minnesota Sen. Amy Klobuchar, South Bend, Indiana, Mayor Pete Buttigieg, Sen. Elizabeth Warren of Massachusetts and Vermont Sen. Bernie Sanders — are debating on Tuesday night.