The conventional wisdom is that Donald Trump only became a conservative the day he announced his candidacy for the presidency. But like all conventional wisdom about Trump, it’s wrong.

After President Obama took office, Trump told me almost eight years ago the new president was a “disaster” whose economic policies were going to ruin the country.

Trump wasn’t ready to be quoted then. But almost five years ago, in a book that has been largely overlooked during the campaign, Trump laid out exactly what’s wrong with Obama’s vision and why conservative policies are needed to turn around the country’s pathetically slow growth under his leadership.

In “Time to Get Tough: Making America #1 Again,” which came out in December 2011, Trump presented a detailed economic critique that any fiscal conservative would applaud.

The reason “this country is an economic disaster right now,” he wrote, “is because Barack Obama doesn’t understand how wealth is created — and how the federal government can destroy it.”

Liberals “scratch their heads and wonder why businesses don’t want to hire,” Trump wrote. The answer: “Companies know Obama is anti-business, and his government-run health-care takeover has created a major disincentive to hire new workers.”

Raising taxes, as Obama wants to do, merely forces business owners to “lay off employees they can no longer afford,” Trump noted. “It also drives up prices, encourages businessmen and women to move their businesses (and their jobs) to other countries that have far lower tax rates and regulatory costs, and sends people scrambling for tax shelters.”

Conservative though he is, Trump knows how to appeal to most Americans. As Norma Foerderer, Trump’s top aide for 26 years, told me, there are two Donald Trumps: the “outrageous” one portrayed on television and the real one only insiders know.

The private Donald Trump, on the other hand, is “the dearest, most thoughtful, most loyal, most caring man,” Foerderer said.

Illustrating the difference, last summer, the Hispanic Chamber of Commerce, which represents 3.2 million business owners, announced its members would be boycotting all of Trump’s properties following his statements on illegal immigrants and his vow to build a wall across the entire Mexican border. But last September, Trump met privately with Javier Palomarez, the chamber’s CEO.

“There were no bombastic statements of any sorts,” CNN quoted Palomarez as saying admiringly. “It’s kind of interesting, the dichotomy between the private Donald Trump and the public Donald Trump. He listened a lot more than he spoke.”

Far from being a bigot, Trump insisted on admitting blacks and Jews to Mar-a-Lago when several other Palm Beach clubs wouldn’t. When I first got to know Trump while conducting research with my wife Pam for my 1999 book “The Season: Inside Palm Beach and America’s Richest Society,” on the way down to Palm Beach on his plane, Trump imitated the nasal, constricted tones of Palm Beach’s blue-blood Old Guard condemning his club for not discriminating.

If Trump is intemperate, as the conventional wisdom has it, his employees haven’t seen it. Rather, as an employer, Trump is both demanding and loyal, according to Anthony P. “Tony” Senecal, who for 20 years served as personal butler to Trump and is now the Mar-a-Lago historian.

Some years ago, when Senecal had to undergo surgery to implant a stent, Trump called him the day before.

“So when do you go under the knife?” Trump asked.

“Tomorrow,” said Senecal.

“Well, if you don’t make it, don’t worry about it. You’ve had a good life,” Trump said, and then added: “Listen, I don’t want you going back to your place. You come and recuperate at Mar-a-Lago.”

“The guy is fairer than hell,” says Gary J. Giulietti, a Trump friend who handles a portion of his insurance as president of Lockton Cos., the largest privately held insurance brokerage company in the world. “He wants the best for his properties, he wants a competitive price. But he treats everyone with respect.”

The conventional wisdom that Trump is a carnival act will be proven wrong once again when he moves into the White House. Donald already has a winter White House — Mar-a-Lago — picked out.

Ronald Kessler is the author of “The First Family Detail: Secret Service Agents Reveal the Hidden Lives of the Presidents.”