By Jonathan Stempel and Trevor Hunnicutt
NEW YORK (Reuters) - Warren Buffett, the billionaire chairman of Berkshire Hathaway Inc , on Thursday said the conglomerate bought back its own stock for the first time since 2012 and added to its already huge stake in Apple Inc .
Buffett also said investors are better off owning a basket of stocks than long-term bonds as a strong U.S. economy bolsters corporate profits, despite higher costs from tariffs.
"I don't know when to buy stocks, but I know whether to buy stocks," Buffett, celebrating his 88th birthday, said on CNBC.
"Business is good across the board," he added.
Berkshire has more than 90 businesses in the insurance, energy, food and retail, industrial and other sectors, and invests in companies such as Apple, Wells Fargo & Co and Coca-Cola Co .
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A buy-back policy announced in July gave Buffett, who has gone 2-1/2 years since a major acquisition, a new way to deploy Berkshire's $111.1 billion of cash and equivalents.
Berkshire said the policy frees Buffett and Vice Chairman Charlie Munger to repurchase stock when the price was below Berkshire's "intrinsic value," a determination that would be made "conservatively."
The old policy allowed buybacks when Berkshire traded at below 1.2 times book value, but the Omaha, Nebraska-based company's shares have long traded above that level.
Buffett said "we've bought back a little" stock since the change, and that he and Munger need "a big enough discount" to ensure that long-term shareholders are better off.
Berkshire has also bought "just a little" more Apple stock since June 30, he said, when it had a 252 million share stake now worth more than $56 billion.
Buffett is also a happy Apple customer.
He said he uses his iPad "a lot," and that the iPhone is "enormously underpriced" even when it costs $1,000, given how indispensable it has become for many people.
Buffett spoke at the Smith & Wollensky steak house in Manhattan before dining with the person who in June agreed to pay $3,300,100 at an annual charity auction to join him.
At the restaurant, Buffett held court for hours, over a meal that typically includes his signature Cherry Coke and a $59 Colorado rib eye steak. Guests sang happy birthday, and Buffett was given a cake.
Auction proceeds benefit the Glide Foundation, a San Francisco charity that serves people who are poor, homeless or battling substance abuse. The winning bid was the third-highest in 19 years of auctions, which have raised $29.6 million.
(Reporting by Trevor Hunnicutt and Jonathan Stempel in New York; Additional reporting by Suzanne Barlyn; Editing by Steve Orlofsky and Dan Grebler)