Meridian Structural Engineering elected Illinois's pass-through entity tax, and the calendar this tool builds for an S corporation never asks about it

The moment

Meridian Structural Engineering is a nine-person structural engineering firm in Naperville, Illinois, founded by two licensed professional engineers, Dana Okafor and Priya Raghunathan, in 2013. They stamp drawings for warehouses, parking structures and mid-rise additions across the western Chicago suburbs; six more engineers and a project coordinator round out the staff, plus one land surveyor Meridian pays as a contractor rather than an employee for jobs that need a fresh boundary survey. The firm is taxed as an S corporation — Dana and Priya each take a salary and split the remaining profit as an owner distribution, the way most firms this size are set up.

Late last year, Meridian's outside CPA raised something new: Illinois lets a pass-through entity like Meridian elect to pay its own state income tax at the entity level instead of passing it through to Dana's and Priya's personal returns, which changes how much of that state tax rides along with the deduction cap a personal return runs into. Dana and Priya made the election for the 2025 tax year and kept it for 2026. It's the last week of September, and Priya is building next year's filing calendar the way the firm does every fall — one page, every date Meridian owes something to someone, pinned above the printer.

What they did instead

For two prior years, before the election, the calendar was simple: quote the Illinois estimated-tax page, write down four dates, done. This year Priya did the same thing again — opened this tool, told it Meridian is an S corporation, and let it build the calendar — without stopping to ask whether "the Illinois estimated-tax page" the tool was reading from still described the entity Meridian had actually become for state tax purposes the moment the election went in. The calendar came back clean, four Illinois dates among the rest, nothing flagged, nothing missing. A clean calendar looks the same whether it is complete or whether it is answering a question nobody actually asked, and for two years running that difference cost nothing, because the question had never come up before.

The questions a CFO asks first

1. Now that Meridian has made this election, does the state's own published schedule still treat it as an ordinary S corporation for estimated-tax purposes, or does the election move it onto a different one?
2. If the tool's calendar and the schedule Meridian's election actually follows disagree, which date is earlier, and by how much?
3. If Illinois's own sales-tax filing frequency isn't confirmed yet, does the tool guess a schedule or say so plainly?
4. Does the tool ask about the election at all, or does it only show up if someone thinks to check?

The arithmetic, shown

Three runs, all through the tool's own engine, all built on Meridian's actual profile: an S corporation, seven W-2 employees, one 1099-NEC surveying contractor, Illinois added as the state, filing Form ST-1 monthly.

**Scenario one — the full year, run as the S corporation Meridian actually is.** The tool returns twenty-seven dated line items, including four Illinois individual estimated-tax installments on the owners' own return: April 15, June 15 and September 15, 2026, then

> January 15, 2027

for the fourth. That is the correct schedule for an S corporation that has not elected Illinois's Pass-Through Entity tax. It is also the only schedule this tool can produce for an S corporation, elected or not — the tool's own note says why:

> This tool doesn't cover Illinois's Personal Property Replacement Tax rate, the Pass-Through Entity tax election itself, any partnership or individual use-tax reporting rule, or Chicago's or any county's own home-rule sales tax — this run verified only the C corporation's own return and estimated-tax schedule, the individual estimated-tax schedule, and the base sales-tax due dates. An S corporation or partnership that has actually elected Illinois's Pass-Through Entity tax follows the same 'Businesses' estimated-tax schedule as a C corporation, not the individual one below — this tool doesn't ask about that election, so no Illinois date is computed for that case here.

**Scenario two — Illinois's own sales-tax filing frequency not yet confirmed.** Run with everything else answered but Illinois's own average monthly liability tier left blank, the tool does not guess monthly, quarterly, or annual on Meridian's behalf. It returns a named refusal: missing: ["how often Illinois has this business file sales tax"]. Nothing about the Illinois sales-tax side of the calendar computes until an actual answer exists.

**Scenario three — the identical profile, told to the tool as a hypothetical C corporation instead.** Running Meridian's exact facts with only the entity question changed returns twenty-nine dated line items, not twenty-seven, and a different Illinois schedule entirely: no individual estimated-tax dates at all, and in their place, an Illinois corporate return plus four corporate estimated-tax installments on April 15, June 15, September 15, and

> December 15, 2026

The Department's own source names both schedules in the same sentence, not as an inference this tool is drawing on its own:

> Installment dates are the following, unless the date falls on a weekend or holiday, then the payment will be due on the next business day. ... Individuals April 15; June 15; September 15; and January 15 of the following year. Businesses April 15; June 15; September 15; and December 15.

Meridian's own election puts it on the "Businesses" line, per the tool's own note above — the same schedule scenario three computes, not the one scenario one actually shows for the S corporation Meridian is on paper. December 15, 2026 comes and goes a full month before January 15, 2027, the date scenario one's calendar names as Meridian's fourth Illinois payment.

What changed

Priya ran scenario three after the CPA mentioned the word "Businesses" schedule in the same call that first raised the election, curious whether the tool's own S-corporation calendar was already accounting for it. It was not, and the tool's own note said so directly rather than leaving her to guess. Meridian now tracks its fourth Illinois installment off a date the CPA supplied directly — December 15 — on the same one-page calendar, marked by hand as a figure this tool does not compute. Nothing about how much Illinois tax Meridian actually owes changed; what changed is which of the tool's own twelve months Priya double-checks against the CPA's own letter before trusting it blank.

What the tool will not do

It will not ask whether this business has elected Illinois's Pass-Through Entity tax, and it will not compute a date for the "Businesses" schedule for anything but a calendar-year C corporation. An S corporation or partnership that has made this real, common election gets the same calendar as one that has not — complete-looking, not flagged as incomplete, and wrong about which of the state's own two published schedules actually applies to it.

It will not compute the dollar amount Meridian owes Illinois under either schedule, this quarter or any quarter, or verify the Personal Property Replacement Tax that rides along with the corporate return this tool does track dates for.

It will not verify the average-monthly-liability threshold Illinois uses to assign monthly, quarterly, or annual sales-tax filing in the first place — Meridian's own letter from the Department already settled that question for this year; a business asking "will I be assigned monthly" gets no answer here.

It will not know about an extension Meridian files, a payment plan already in place, or a penalty already assessed on any of these dates. This tool supports a named, growing list of states — Texas, Florida, Washington, Nevada, Wyoming, California, New York, South Dakota, Tennessee, Colorado, North Carolina, and Illinois as of this writing — and every line above is the plain, unmodified due date for the entity type this tool was told about, not one adjusted for an election only Meridian's own account with the Department would know it made.