Form 1120-S Deadlines and Extensions
S-corporation returns run on a different calendar than personal returns and the penalties for missing the deadline are per-shareholder, per-month. Here's what to know.
Original deadline
Form 1120-S is due March 15 for calendar-year S-corps. This is one month earlier than personal Form 1040.
The reason: shareholders need their K-1s before their personal returns are due April 15.
Extension
File Form 7004 by March 15 to get a 6-month extension to September 15.
Extension is automatic (no reason required) but must be filed on time.
Extension is to file, NOT to pay — but S-corps generally don't owe federal tax at the entity level, so there's rarely a payment.
Late filing penalty
$245 per shareholder per month (or fraction thereof) the return is late, up to 12 months.
A 3-shareholder S-corp filing 4 months late = $245 × 3 × 4 = $2,940. Ugly.
First-time penalty abatement is often available if you have a clean 3-year compliance history.
Montana specifics
Montana also requires an S-corporation return (Form CLT-4S / PTE) due the same date. Extension via Form CLT-4S EXT.
Montana passes income through to shareholders similar to federal.
What to file with the return
Full Form 1120-S with balance sheet, income statement, and reconciliation.
Schedule K-1 for each shareholder.
State returns for every state where the S-corp has nexus.
Form 8879-S signed authorization for e-filing.
A quick disclaimer
This article is general information for Montana small business owners, not tax, legal, or accounting advice for your specific situation. Rules change, and how they apply depends on facts we don't know about you. Before acting on anything you read here, talk to a qualified professional. If you're a Montana business owner and want a real conversation about your books, payroll, or tax, that's what Marlow Accounting is here for — call 406-290-1214 or schedule a discovery call.