More than a year after passage of the 2017 tax overhaul, multinational companies like Nike Inc. and Walgreens Boots Alliance, Inc. still aren’t sure what the new rules mean for their bottom lines—setting up a bumpy ride through the upcoming earnings season.

Companies are still waiting for the government to answer questions about the law’s international provisions so they can calculate their tax bills properly. At the same time, an SEC rule that let them take a pass on reporting some post-overhaul tax numbers has just expired, so they have to report something.

They’ll probably have to keep adjusting those numbers throughout the year to keep investors informed. That volatility could make investors nervous, and could even affect earnings per share, if a company’s after-tax income changes, said Andrew Silverman, a Bloomberg Intelligence tax policy analyst.

“Companies will not get their taxes correct this year,” Silverman wrote in an email to Bloomberg Tax. “They don’t have enough information from the IRS and Treasury to get them correct.”

And the partial government shutdown didn’t help either, after the IRS released an avalanche of guidance at the end of 2018.

“By the time all that got digested, many of the Treasury people, who we would submit comments to or who would participate in panels, those people were unavailable,” said David Sites, an international tax partner at Grant Thornton.

Nike, Walgreens, and other companies including Bed Bath & Beyond Inc. and Meredith Corp. have said in recent filings and investor calls that they were still calculating the effects of certain provisions, or expected to adjust their reported tax assessments as regulations come throughout 2019.

“The finalization of regulations related to U.S. tax reform may result in discrete adjustments that impact our tax rate,” Andy Campion, executive vice president and chief financial officer at Nike, said on a Dec. 20 call with investors.

The Securities and Exchange Commission had given companies a year from the law’s enactment to work through their tax assessments. But that notice, Staff Accounting Bulletin 118, expired in December.

“This is going to be a roller coaster of a year, as companies react to tax guidance and change their numbers,” Silverman added.

Here are some of companies’ biggest questions about the law.

Global Intangible Low-Taxed Income

GILTI has been one of the most highly scrutinized international provisions of the tax law. It ensures that corporations pay at least some tax on their foreign income by imposing a 10.5 percent rate on profits above a deemed rate of return that aren’t already taxed at a certain threshold.

Even though the IRS released guidance on GILTI income under tax code Section 951, the complex rules are still tripping up companies as they wait for final guidance. Companies are looking for more information on how GILTI will affect their tax planning, Silverman said, including how they can use tangible assets in their foreign companies to reduce their GILTI, or maximize foreign tax credits from high-tax regimes to cancel out GILTI tax from regimes with lower rates.

GILTI is “probably the most common one that’s cited in SEC filings,” said Ron Graziano, director at Credit Suisse Accounting & Tax Research. “If a company comes out definitively and says, ‘our rate is increasing for the foreseeable future because of the application of GILTI,’ that’s something that could definitely influence investors’ take on a company.”

Companies said in recent filings that they were still trying to calculate GILTI’s effect on them.

“Given this complexity, we have not made any adjustments related to a potential GILTI tax in our financial statements and have not made a policy decision regarding whether to record deferred taxes on GILTI,” Meredith wrote in a Jan. 8 filing.

And Bed Bath & Beyond said in a Jan. 9 filing that, “Due to the complexity of these provisions, the Company is not yet able to reasonably estimate the long term effects of GILTI.”

Foreign-Derived Intangible Income

The 2017 tax law was intended to encourage companies to bring assets back to the U.S., and the FDII provision is one of the sweeteners that tries to achieve that aim. The provision, Section 250(a), lets companies lower their tax rate by providing a deduction for income earned from foreign sales or services above a deemed 10 percent rate of return on tangible assets—an incentive for companies to keep their intellectual property in the U.S. and export goods and services.

Industrial and construction materials manufacturer Chase Corp. said in a filing it was waiting on proposed rules for FDII before applying the deduction to its current period. FDII guidance has been under review at the Office of Management and Budget since before the shutdown.

“Probably the biggest regulatory need is for guidance on the FDII rules,” said John Warner, a shareholder at Buchanan Ingersoll & Rooney PC.

That’s because the statute imposes specific requirements on corporations trying to take advantage of the deduction, but doesn’t clearly explain them, he said. For example, companies are waiting for Treasury to explain precisely what “sold for foreign use” means, and how to establish whether the buyer is a foreign person, Warner said.

Repatriation Tax

Under Section 965, U.S. multinationals are required to pay a one-time “transition tax” on income accumulated overseas since 1986. The income is deemed to be repatriated.

Companies have already calculated how the tax will hit them, using proposed regulations released last year. But they may have to redo their math since Treasury released final rules Jan. 15.

The rules mostly adopt a proposal the IRS released in August 2018, but they don’t address a controversial issue dealing with overpayments.

Companies can elect to pay their repatriation tax in eight installments, but the IRS has said companies won’t get refunds for any excess payments. Instead, those extra funds will be applied to the next installment. It didn’t change its mind in the final rules.

“Companies need to understand what is the final result of the 965 inclusion,” Joshua Odintz, a partner at Baker McKenzie LLP, told Bloomberg Tax before the final rules were released. Companies could end up having to pay additional tax, or applying for a refund, on their previous year’s return based on the final rules, he said. “That’s an issue that could be sizable, depending on the issues at play.”

In September, FedEx Corp. warned investors that it may reverse a previously reported $225 million benefit if the final repatriation rules matched the proposed version. In December, the company said it would go to court to challenge Treasury’s interpretation of the statute.

“Certain guidance included in these proposed regulations is inconsistent with our interpretation that led to the recognition of a $225 million ($0.94 per diluted share) benefit in 2018,” the company said in a Dec. 18 filing.