America’s new tax system will go into effect in just 12 days, and payroll companies are bracing for confusion as they figure out new withholding rules that will affect millions of American paychecks.

The Treasury Department and the IRS will have to quickly write new regulations to implement the new law, governing everything from the tax regime for businesses that don’t organize as corporations to the endowments of the nation’s elite universities and how multinational corporations are taxed on the profits they make abroad.

And while the vast majority of taxpayers would see a tax cut next year, Americans who are considering selling real estate or other types of capital assets, paying property taxes, taking out a mortgage or incorporating their businesses will have to quickly calculate whether these actions will cost them more or less in the coming year.

Already, state and local officials in high-tax states like New York and New Jersey are coping with a wave of questions from homeowners wondering whether they can prepay their 2018 property taxes in the next two weeks to escape new limits on deductions.

“There will always be bumps in implementation with the IRS, and probably technical corrections to follow,” said Sen. John Thune of South Dakota, a member of GOP leadership and a key negotiator on the tax bill. “There usually are with big bills like this.”

House Ways and Means Chairman Kevin Brady (R-Texas) has also acknowledged that lawmakers will almost certainly need to pass corrections to the tax bill, H.R. 1 (115), which took less than seven weeks from the first bill’s introduction to final passage. Brady said Monday that he started meeting with Treasury Secretary Steven Mnuchin last week to start figuring out how the administration can flesh out the tax bill wherever necessary.

But while Republicans will continue to plug this new tax code as a victory for the American worker, they shouldn’t expect any help from Democrats in helping to smooth those bumps or fix the inevitable mistakes that are being written into the code.

“If they do pass the bill, they lose with the American people,” House Minority Leader Nancy Pelosi of California told reporters recently, before talking up Democrats’ chance to regain control of Congress. “Because even when we do win, to undo this will cost a fortune.”

The most complex of the new rules, such as those mandating new relief for so-called pass-through businesses and how to tax offshore business profits, are expected to take years for Treasury and the IRS to write. In the long run, the GOP also will have to grapple with the temporary provisions they decided to write into their own bill, which ends tax relief for individuals after eight years.

More immediately, taxpayers in high-tax states are rushing to see whether they can seek extra relief ahead of a new cap on the state and local tax deduction that will go into effect on Jan. 1. Taxpayers will get to deduct only up to $10,000 in both property and income or sales taxes going forward, and GOP negotiators specifically banned people from prepaying 2018 income taxes this year for an extra tax break.

But the final compromise tax bill doesn’t forbid taking a deduction this year for prepaying property taxes, though accountants in California, New Jersey and New York caution that pulling off such a move won’t be so easy in some places.

“The number of calls this year on the issue has clearly seen a spike because of what’s going on with federal tax reform,” said Peter Baynes, the executive director of the New York Conference of Mayors.

Homeowners whose mortgages require property tax payments to be held in escrow could have trouble with their banks, and one New Jersey accountant suggested taxpayers might have a problem taking a property tax deduction for a payment when no bill is due.

In New York, individual jurisdictions must take special steps to allow their tax collector to start accepting early property tax payments, a move already taken by some municipalities.

But while that issue will be moot in less than two weeks, tax experts say that other problems raised by the bill could vex Treasury and the IRS for far longer. At least at the start, the IRS will also be taking on its rule-writing task without a permanent commissioner, and after years of funding declines that have raised questions about the agency’s ability to shoulder all of its new work.

Tax experts have long questioned whether the government could adequately protect a special tax break for businesses that pay taxes through their owners’ individual returns, called pass-throughs, from being abused.

Thirteen law professors, at least a couple of whom worked for the Obama administration, recently detailed several ways they thought the tax bill’s 20 percent deduction for such income could be gamed. The GOP plan, they wrote, would give taxpayers an incentive to become independent contractors instead of employees to earn tax relief.

The IRS could try to limit wealthy professionals like doctors and lawyers from using that deduction, but the law professors believe there are plenty of ways that creative accountants and lawyers could work around those restrictions.

Multinational corporations have similar incentive to game new international tax rules like a new minimum tax on global income, while experts are also worried that a tax meant to limit offshore gaming would run afoul of the WTO.

“Some of these issues they’re going to run into are just technical problems,” said Marty Sullivan, the chief economist at Tax Analysts. “But others are aggressive moves, where we’re going into the unknown in areas like international and on pass-throughs and doing things we’ve never done before.”

On top of that, Steve Rosenthal of the Urban-Brookings Tax Policy Center noted that the IRS would be allowed to write rules retroactive to Jan. 1 for only 18 months except in cases of abuse, putting the cash-strapped agency on a tight timeline.

The GOP plan’s individual tax relief also expires at the end of 2025, raising the possibility of a sequel to the fiscal cliff that Washington endured at the close of 2012 — though Republicans say they’ll work hard to avoid that outcome.

“We have every intent of making those permanent,” House Speaker Paul Ryan said Tuesday.

Larry Stone, a longtime Democratic fundraiser who is now the county assessor in Santa Clara, California, said taxpayers should talk with an accountant before acting on the tax bill.

“The problem is, this thing is being changed almost by the hour, and we’re getting close to the end of the year,” he said.

Keshia Clukey, Marie French, Ryan Hutchins and David Siders contributed to this report.