Post-Closing Compliance: Critical Deadlines in Your First 90 Days

Acquiring a business creates immediate tax and compliance deadlines—EIN registration, payroll setup, estimated tax payments, and entity elections—that must be met to avoid penalties and unfavorable tax treatment. The first 90 days after closing are more procedurally intensive than most buyers expect.

Obtain a new EIN. A new entity needs its own Employer Identification Number. If you acquired assets (not stock), you need a new EIN even if the business name stays the same. The IRS Form SS-4 application is straightforward but must be filed before payroll or bank accounts are set up.

Payroll registration. You must register for federal and state payroll tax accounts, set up withholding, and begin depositing employment taxes on the IRS schedule that matches your anticipated liability (monthly or semi-weekly under Section 6302).

Sales tax permits. Each state where you have nexus requires a separate sales tax permit. Selling without a permit can trigger penalties even if you collect and remit the tax.

Estimated tax payments. Your first quarterly estimated payment (Form 1040-ES or 1120-S) may be due before your first profitable month. Missing the deadline triggers underpayment penalties under Section 6654.

Entity election deadlines. S corp elections (Form 2553) must be filed within 75 days of formation to be effective for the first tax year.

The pitfall: Missing a single post-closing deadline -- especially the S corp election or first estimated payment -- can lock you into an unfavorable tax structure or trigger penalties that erase months of profit.