Because the tax overhaul — the first of its scope in more than three decades — is coming so late in the year, it is setting off a mad dash by public companies. They are required to report the effects of the new law in their financial statements to shareholders in the same quarter that the law becomes enacted, even if the measures themselves go into effect later.

Finishing the necessary analysis on time will be tough. Companies whose fiscal year closes on Dec. 31 or Jan. 31, as is the case for many retailers, will have just a few weeks to produce quarterly and annual financial statements for investors that reflect the new tax system.

Expect turbulence.

“Maybe 50 percent of our clients were following the debate and knew it was headed this way, but I’ve been surprised by the numbers of clients who, by their own admissions, didn’t think this would happen,” said Kate Barton, the vice chairwoman of tax services for the Americas at Ernst & Young. “Maybe 30 to 40 percent of the heads of tax at corporate clients are scrambling to model out the new law and its impact on their businesses.”

A recent Ernst & Young webcast about tax changes for wealthy clients drew 4,671 people. One this week for corporate clients drew more than 12,000 participants, Ms. Barton said.

All this is a headache — possibly a migraine — for many companies and individuals.

For many tax professionals, it’s thrilling.