Every entity form maps to one federal return and one hard deadline. The cost of missing a pass-through deadline is not a percentage of tax owed — it is a flat, per-owner, per-month penalty that runs whether or not the entity earned a dollar. Know your form and your date before the year begins.
The penalties that hurt are the pass-through ones. A late or missing Form 1065 or 1120-S draws a penalty of roughly $235 per partner or shareholder per month, up to twelve months, under IRC §6698 and IRC §6699 — a four-owner partnership that files five months late owes about $4,700 before anyone computes a dollar of income tax, and each late K-1 to an owner carries its own information-return penalty on top. Pass-throughs can extend six months on Form 7004, but an extension moves the filing deadline, not the owners’ payment deadline; tax still accrues interest from the original date.
Employment and information returns run on a separate calendar. Payroll returns (Form 941 quarterly, Form 940 for federal unemployment annually) and the W-2 and 1099 information returns covered in section “Paying for Work: Payroll and Information Reporting” are due January 31, independent of the income-tax return. The state layer — franchise tax, sales tax, and the state income return — carries its own deadlines, usually but not always tracking the federal date. Put every recurring date on a calendar with two weeks of buffer; the penalties above are pure deadweight, entirely avoidable, and routinely paid by businesses that simply lost track of the date.