Hillary Clinton backer Warren Buffett told CNBC on the eve of Donald Trump 's inauguration that the billionaire real estate mogul has the "most important job in the world."

"America works," the chairman and CEO of Berkshire Hathaway (BRK-A) said. "I've said this before. It'll work wonderfully under Hillary Clinton, and I think it'll work fine under Donald Trump."

At a red carpet event Thursday night in New York for a new HBO documentary about his life, Buffett told CNBC the United States has the "secret sauce."

"It doesn't work all the time perfectly," the billionaire investor and philanthropist said, "but you just look at where we go, milestone after milestone. Never bet against America."

With that mantra as a guiding principle, Buffett said, "we're almost always a buyer of stocks over time."

Reiterating his call to invest for the long haul, Buffett acknowledged that he doesn't know where the stock market will go in the next "10 days or a year or two years."

But one thing he said he does know: "It's going to be higher 10 years, 20 years from now."

"There will be hiccups from time to time in the economy," he cautioned. "[But] we'll do well over time."

The HBO documentary "Becoming Warren Buffett," premieres on Jan. 30. It tells the story of the Oracle of Omaha's rise to become one of the most revered businessman in the world.

Buffett told CNBC: "This is a story about my dad and my first wife, and also about my second wife, who all really put me together, saved me. So I think people will see a different side of me than they've seen before."

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