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The data behind this site

By Brent Chudoba, editor Updated

Every rate, fee, deadline and threshold on Cobalt Prosper comes from one of the 15 datasets below. Each one is collected from primary government sources, graded field by field for how directly the source confirms it, and checked by an automated gate before anything publishes. They are free to download and use as JSON. If you cite a figure, please link the page or the dataset and give the date it was reviewed.

How the data is kept honest

Datasets

Cobalt Prosper datasets
DatasetRecordsLast reviewedRefreshedDownload
County property tax calculation detail 15 August 30, 2026 Every 12 months (March) JSON, 14 KB
County property tax due dates, billing and appeals 500 September 24, 2026 Every 12 months (August) JSON, 2,089 KB
County property tax figures (Census ACS) 3,144 September 23, 2026 Every 15 months (December) JSON, 2,074 KB
County sales tax rates 3,133 September 9, 2026 Every 3 months (January, April, July, October) JSON, 4,869 KB
LLC formation by state: steps, costs and first-year obligations 5 September 24, 2026 Every 12 months (January) JSON, 93 KB
Sales tax economic nexus thresholds 47 August 30, 2026 Every 12 months (February) JSON, 34 KB
State college aid programs 10 September 7, 2026 Every 12 months (November) JSON, 29 KB
State employer and sales tax registration 51 September 8, 2026 Every 12 months (January) JSON, 834 KB
State FAFSA deadlines 51 August 30, 2026 Every 9 months (November) JSON, 21 KB
State income tax rate comparison 51 August 30, 2026 Every 12 months (January) JSON, 19 KB
State income tax, retirement tax and LLC facts 51 August 29, 2026 Every 12 months (January) Not published separately
State property tax rules 51 September 7, 2026 Every 12 months (February) JSON, 239 KB
State sales tax rates 51 August 30, 2026 Every 12 months (February) JSON, 33 KB
US income tax treaties 68 September 15, 2026 Every 12 months (October) JSON, 286 KB
US tax residency and foreign earned income rules 7 September 16, 2026 Every 12 months (November) JSON, 263 KB

County property tax calculation detail

The worked example and rate breakdown on the counties we have detail for. 15 records. Last reviewed August 30, 2026.

Why it goes out of date: Millage rates are set annually by each county, and assessment ratios change by state law.

Recent changes:

County property tax due dates, billing and appeals

The billing, due-date, appeal and county relief sections on county property-tax pages that have a verified record (206 counties: 6 pilot plus the top 200 by search impressions). 500 records. Last reviewed September 24, 2026.

Why it goes out of date: Not all at once, and that is the point of the per-field `changes` grade. Due dates and relief dollar amounts are `annual`: they reset every tax year and most counties post the new calendar between July and October. Appeal windows and installment plans are `occasional`: they move with an ordinance or a law change. Office names and URLs are `stable`: they only need a dead-link check. The gate warns when every due date on a record has passed, which is the signal that the next cycle needs collecting.

Recent changes:

County property tax figures (Census ACS)

All 3,133 county pages and the 51 state ranking pages. 3,144 records. Last reviewed September 23, 2026.

Why it goes out of date: Census ACS releases new five-year estimates annually, usually in December. Everything in this file moves together on that one clock: the headline median bill and value, the bill bands, the mortgaged/owned-outright medians, owner income and tenure all come from the same vintage, and the five-year comparison moves its prior period forward in step. The county adjacency file changes only when county boundaries do (Connecticut's 2022 switch to planning regions is the recent example).

Recent changes:

County sales tax rates

3,133 county sales tax pages and 51 state index pages. 3,133 records. Last reviewed September 9, 2026.

Why it goes out of date: Sales tax rates change quarterly in most states and this is the fastest-rotting dataset in the repo, faster than state-compliance. But the cadence is NOT uniform and a single clock will either miss changes or re-collect for nothing. Quarterly: CA, NC, TX, OH, KS, MO, OK, ND, MN, UT, AR, GA. Semiannual: IA (Jan and Jul only), CO (Jan-Jun and Jul-Dec editions with a URL that MOVES each time). Annual: NM (July to June since 2025, no mid-year changes). Referendum-driven and unschedulable: WY, whose county taxes expire and are re-voted, and VA, where the 2026 Appropriation Act let all 133 localities adopt an extra 1% by referendum on a date of their own choosing. Two states publish only the FUTURE edition, so the current one is unobtainable: MN deletes the superseded guide, and WV's municipality list is headed months ahead.

Recent changes:

LLC formation by state: steps, costs and first-year obligations

The filing steps, full costs, name rules, first-year obligations and official-guidance corrections on the LLC formation page for each state with a verified record (5 pilot states: Alaska, Georgia, New York, Utah, Virginia). 5 records. Last reviewed September 24, 2026.

Why it goes out of date: Filing fees move by statute or fee regulation, usually effective July 1 or January 1. Portals get redesigned, which breaks the step descriptions without changing any fee. The federal beneficial ownership rule has changed several times since 2024, and state help pages lag it by months. The weekly source watch flags a changed cited page before the annual refresh would.

Sales tax economic nexus thresholds

The economic nexus section on each sales-tax state page, and the nexus checker. 47 records. Last reviewed August 30, 2026.

Why it goes out of date: States keep dropping the transaction-count prong of their thresholds. Several have done so in the last three years.

Recent changes:

State college aid programs

State aid mechanism prose on the FAFSA state pages. 10 records. Last reviewed September 7, 2026.

Why it goes out of date: Programme dates move annually and programmes get defunded. New Hampshire's Governor's Scholarship is still in statute and still listed by the state while carrying no appropriation, which is exactly the failure this dataset exists to catch.

Recent changes:

State employer and sales tax registration

Multi-state employer and sales-tax registration obligations. 51 records. Last reviewed September 8, 2026.

Why it goes out of date: Every state resets its unemployment new-employer rate and taxable wage base each January, and paid-leave contribution rates are reset each autumn for the following year. This is the fastest-rotting dataset in the repo: it is materially wrong within twelve months.

Recent changes:

State FAFSA deadlines

51 state FAFSA deadline pages plus the deadline hub and tracker. 51 records. Last reviewed August 30, 2026.

Why it goes out of date: Deadlines change every award year and the cycle turns over mid-year, so the data can be individually valid and collectively useless. The gate now fails once most dated deadlines are in the past unless the file declares the next cycle.

Recent changes:

State income tax rate comparison

State income tax comparison pages. 51 records. Last reviewed August 30, 2026.

Why it goes out of date: Same annual inflation adjustment and rate changes as states.json.

Recent changes:

State income tax, retirement tax and LLC facts

State income tax pages, the take-home calculator and several explainers. 51 records. Last reviewed August 29, 2026.

Why it goes out of date: Income tax brackets and standard deductions are inflation-adjusted annually and several states are mid-phase-down of their rates.

Recent changes:

State property tax rules

The 'Property tax rules in <state>' section on every county page. 51 records. Last reviewed September 7, 2026.

Why it goes out of date: Exemption amounts are inflation-indexed in several states and legislatures change them every session. BACKLOG.md lists the specific time bombs, including Indiana's homestead deduction phasing to zero by 2030 and Kentucky's biennial reset.

Recent changes:

State sales tax rates

51 state sales-tax pages and the sales tax calculator. 51 records. Last reviewed August 30, 2026.

Why it goes out of date: State and local rates change by legislation and local ballot measure. The average local rate is a statewide average that drifts continuously.

Recent changes:

US income tax treaties

The /taxes/tax-treaties/ index (67 countries) and one page per treaty partner with verified article data (11 as of 2026-09-16). 68 records. Last reviewed September 15, 2026.

Why it goes out of date: Treaties themselves change rarely, but when they do the change is total: a protocol replaces a whole article (Spain 2019 and Japan 2019 both took interest to 0%), a treaty is suspended (Russia 2024), or a new one takes effect (Chile 2024). IRS Table 1, the obvious shortcut, was still showing Japan's pre-2019 10% interest rate when read in September 2026, so it cannot be the refresh source. The shared US-law facts (Form 8802 user fee, form revisions) move on the IRS's own schedule.

Recent changes:

US tax residency and foreign earned income rules

7 pages: the substantial presence test, Form 8840 closer connection, the foreign earned income exclusion, physical presence, bona fide residence, tax home, and resident vs nonresident alien status. 7 records. Last reviewed September 16, 2026.

Why it goes out of date: The exclusion and the housing amounts are inflation-adjusted every year and are set by a Revenue Procedure in the autumn, long before the IRS revises Publication 54 or the Form 2555 instructions. Publication 54 is now a continuous-use guide that prints no amounts at all, and the Form 2555 in circulation lags a year, so the ONLY current source is the Revenue Procedure and the housing Notice. The physical presence waiver country list is a separate annual Revenue Procedure that runs a year behind, and the rules themselves barely move, which is what makes a stale amount easy to miss.

Recent changes:

Using the data

The files are plain JSON with the source URL, confidence grade and review date stored next to each value, so you can check any figure against its source yourself. Found something wrong or out of date? Tell us; corrections are how a source of record stays one.

Nothing here is financial, tax or legal advice. The data describes what government sources say; it does not tell anyone what to do.