U.S. Department of the Treasury Internal Revenue Service 1040 Individual Income Tax forms. (Daniel Acker/Bloomberg)

Republicans in Congress celebrated the passage of the biggest rewrite of the U.S. tax code in decades Wednesday, with President Trump calling it a “Christmas gift for hard-working Americans.” Workers will see the first glimpse of a tax cut in February at the earliest, but it won’t be until 2019 — when people file their taxes for next year — that most will know whether they will pay more or less to the federal government.

In the meantime, tax attorneys, accountants and corporate payroll departments are scrambling to adjust to changes that won’t be official until Trump signs the bill in January.

Homeowners are already peppering local officials with questions about whether they can — and should — prepay their 2018 property taxes in the less than a dozen days left in 2017, before new limits on such deductions take effect. Others are asking their accountants whether there are expenses — such as moving costs — that they should pay now before they are no longer eligible for preferential tax treatment.

Much of the confusion is the result of how quickly Republican lawmakers were able to pass the legislation.

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“I have been a tax attorney over 40 years. I have seen plenty of tax bills. This is unprecedented,” said Robert Willens, a New York-based tax consultant.

Steven Rosenthal, a senior fellow at the nonpartisan Tax Policy Center, was just one year out of law school in 1986 when Congress last passed a significant revision of the tax code. He, and hundreds of other tax attorneys, spent years watching that process unfold, Rosenthal recalled.

This time, “there hasn’t been the deliberate and slow unfolding [of the legislation] like there was in ’86,” he said. “The whole process has been so opaque and so rushed that few people understand what has been going on.”

The new tax legislation, which delivers deep and permanent tax cuts to corporations and temporary relief to individuals, was hammered out by Republicans in a matter of months, delivering Trump his first major legislative victory on an issue many expected would not be resolved until 2018, if at all.

It is a double-edged sword for some business owners. Griff Paper and Film, a 100-person distributor in Philadelphia, plans to take advantage of the favorable tax treatment the law provides for “pass-through” corporations and use the savings to buy new equipment, hire more employees, and train the company’s interns to become long-term workers.

But the slow rollout is proving problematic. The Internal Revenue Service has yet to issue guidance on how the company should handle its payroll, for instance.

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“They screwed it up. It should have come out earlier,” said Chief Financial Officer Mike Trapp.

With little more than a week left in 2017, some individuals are racing to decide whether to take advantage of the changes before the IRS has even finalized the details. The new tax legislation, for instance, would cap deductions for state, local and property taxes at $10,000, prompting a Montgomery County lawmaker to urge the county to help residents prepay their property taxes before the end of the year.

Tax attorneys, meanwhile, said they have heard from clients who plan to accelerate charitable contributions this year because they might not need the deduction next year to lower their tax bill. Others are pondering whether to delay receiving income until 2018, when it could potentially be taxed at a lower rate.

For many people, the law renders obsolete the calculations companies use to withhold taxes from their paychecks, potentially leading some workers to not set aside enough for federal taxes and face a penalty. The American Payroll Association is warning of a potential “disaster” as companies wait to update their computer systems to reflect the changes.

ADP, one of the largest providers of payroll services, says it built its system to quickly reflect changes in the tax rate. But this tax overhaul is more complicated, said Pete Isberg, ADP’s vice president of government relations. Millions of workers have filed W-4 tax forms telling their employers how much in federal taxes they need to withhold. But that form will not be applicable under the new law, Isberg said.

“Therefore, every W-4 on file, all 164 million of them, won’t be valid starting next month,” he said. “We believe every working person is going to need a new W-4.”

The IRS may have to design an entirely new W-4 form, which could take months, Isberg said. To complicate matters further, some states have separate W-4 forms that may also have to be changed, he said. “We need guidance as soon as possible,” he said.

ADP has asked the IRS whether it can automatically convert old W-4 forms into new documents without requiring employees to fill out anything, but the company has not received a response. “We’re looking at ways that we can simplify this for employers,” he said.

A team of dozens of H&R Block executives closely monitored the legislation’s progress, and the company now expects to spend the next year updating its software to reflect the new law in time for the 2018 tax season, said Mark Ciaramitaro, the company’s vice president of tax strategy. The company has already begun developing a training program for its front-line advisers.

“We probably know enough to give people advice, and then if we find out the IRS is interpreting something differently, we can update that training” and alert customers to the changes, he said.

On Monday, PenSoft, a provider of payroll services to small- and medium-size companies, delivered its 2018 software to thousands of customers — just in time for it to become out-of-date.

“There is a misperception that as soon as that bill is enacted, I hit an easy button and the software is updated,” said Stephanie Salavejus, chief operating officer of the Virginia-based company. “But I have to get those formulas from the IRS. I am sort of in a holding pattern.”

The chaos is likely to take months — and perhaps longer — to sort out as the IRS begins writing the rules governing the law’s implementation. The agency has already said it doesn’t expect the tax tables helping employers decide how much in federal taxes should be withheld from workers’ paychecks to be ready until mid-January, allowing them to be implemented in February.

“The IRS will be working closely with the nation’s payroll and tax professional community during this process,” the agency said in a statement.

But some tax experts are also concerned about the IRS’s ability to quickly address the mounting concerns. The agency has been attacked by Republicans for years and has seen its budget cut repeatedly, leading some to question whether it will be up to the task. When the IRS’s former commissioner, John Koskinen, stepped down last month, he blamed Congress for underfunding the agency.

“I don’t know how the IRS is going to enforce this stuff. They have to write regulations, give guidance to taxpayers. They are probably going to feel the brunt of this more than anyone,” said Willens, the tax attorney.

Any delay could sow doubts among taxpayers, many of whom, polls show, are already suspicious of the legislation’s benefits.

“Some employees are going to be anticipating that in their first paycheck in January, they are going to see this big tax cut, and that is not going to happen,” said PenSoft’s Salavejus. “There is still a lot of work to be done.”