President Donald Trump signed the $1.5 trillion tax bill into law on Friday, Dec. 22, slashing rates for American corporations and the wealthy and potentially causing compliance headaches for U.S. businesses and their accountants and financial advisers.

The Senate passed the Republican-led bill legislation Wednesday morning and the House then voted on it for a second time following a technical snafu. The law marks the first major legislative victory for the Trump administration and also the biggest tax overhaul in a generation.

Under the new law, the corporate tax rate is cut to 21% from 35%, which is expected to benefit automakers, banks and oil and gas producers. Overall, the bill could boost earnings for S&P 500 companies by 9.1 percent, according to UBS equity strategists.

Signing the bill before the New Year could also pose at least one problem for corporations. Accounting rules require that the effect of the new law be reflected in the quarter the bill was enacted, said Robert Herz, a former chairman of the Financial Accounting Standards Board who's now at Columbia University's business school in New York.

That means that companies will have to assess the new law's effects on their fourth quarter 2017 financial statements, Herz said. The two biggest issues will be recalculating the value of deferred tax assets at the new corporate rates and, for companies with significant operations overseas, accounting for the repatriation tax they may be liable for regarding cash held outside the U.S.

"That could be a complicated calculation and could be a hit to fourth quarter earnings," Herz said. "Tax accountants, financial reporting folks and CFOs are going to be pretty busy the next few weeks."

The largest U.S. banks will see an average 13% increase in earnings per share from a drop in the tax rate to 21%, according to Goldman Sachs. Wells Fargo & Co. WFC and PNC Financial Group PNC may have the biggest gains, the firm said.

Health care companies, on the other hand, may suffer because the new law eliminates the mandate under the Affordable Care Act that requires Americans to have health insurance or pay a penalty. The measure will also save about $314 billion but means that about 13 million mainly young or poor people will likely go without health insurance.

The new tax law lowers the individual tax rates, including the top bracket to 37% from 39.6%, while doubling the standard deduction. It is expected to widen the gap between the wealthiest Americans and the rest of the country, as it also exempts larger inheritances from the estate tax, doubling thresholds to $11 million for individuals and $22 million for married couples.

An analysis by the nonpartisan Tax Policy Center found that taxpayers in the top 1% of the income distribution (those with income more than $733,000) would receive an average cut of $51,000, or 3.4% of after-tax income. For the middle quintile of earners, the average tax cut would be $930. Over half of the bill's total benefits would go to the top 10 % of earners.

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