Data sources
Every rule in Tax Days is grounded in primary source material — the actual statute, regulation, or treaty text — supplemented with official guidance from the relevant tax authority. We prioritize:
- Statutory text — the actual law, e.g., New York Tax Law § 605(b)(1)(B) for the NY 184-day rule, Internal Revenue Code § 7701(b) for the US Substantial Presence Test, Article 4 B of the French General Tax Code.
- Regulations and administrative guidance — IRS publications (519, 8840 instructions), HMRC RDR3 guidance for the UK Statutory Residence Test, official tax-authority websites.
- Tax treaties — published treaty text and official commentary, particularly for tie-breaker analysis.
- Case law — tribunal and court decisions that clarify ambiguous statutory language (e.g., Matter of Gaied for the NY permanent place of abode test).
Where authoritative material is unclear or the rule is contested, we err toward conservative treatment — assume the rule applies when reasonably ambiguous, and flag the ambiguity in the rule notes within the app and on the per-jurisdiction page.
How we count a "day"
Different jurisdictions count days very differently. A 30-minute layover at JFK is a New York day. A flight that lands in London after midnight may or may not count under the UK SRT depending on the deeming rule. Schengen counts both arrival and departure days. Tax Days encodes these subtleties:
- "Any portion of a day" jurisdictions (NY, NJ, MA, CT, CA, etc.): a calendar day with any presence counts as a full day.
- "Midnight presence" jurisdictions (UK SRT primary rule): a day requires presence at midnight ending that day, with deemed-day exceptions.
- Transit-day exceptions: where a jurisdiction's statute or guidance excludes pure-transit days (no immigration clearance, no overnight stay), Tax Days respects the exclusion.
- Rolling vs. fixed periods: Schengen 90/180 is a rolling window; the SPT is 3-year weighted; most country rules are calendar-year. Tax Days encodes the period type per rule.
Update cadence
Tax law changes. Some examples that have driven recent updates:
- Massachusetts Millionaire's Tax — 4% surtax effective 2023.
- Portugal NHR — closed to new applicants at the end of 2024; replaced by IFICI.
- UAE tax residency tests — Cabinet Decision 85/2022 codified the 90/183-day pathways.
- US Substantial Presence Test — base rule unchanged; closer-connection exception (Form 8840) updated annually with new thresholds.
We track legislative changes from official tax-authority announcements, treasury releases, and the major international tax-news services. When a material rule change occurs, we ship an app update with the new rule definitions and email Pro subscribers a summary.
What Tax Days is not
We're explicit about scope:
- Not tax advice. Tax Days is general information and a tracking tool. It is not a substitute for advice from a qualified tax professional in your jurisdictions.
- Not a complete tax-planning solution. We track residency thresholds. We do not prepare returns, compute exit-tax exposure, or model treaty positions for specific facts.
- Not a guarantee of audit outcome. A contemporaneous day-count log materially helps any audit defense, but auditors examine the totality of facts. Tax Days produces evidence, not certainty.
- Not exhaustive. We track 80+ rules across 250+ jurisdictions. Some niche regimes (provincial sub-rules, special expat regimes with bespoke conditions) require consultation with a local advisor.
Who builds Tax Days
Tax Days is built by Parra, an independent software company based in the United States. We don't take advertising, don't sell user data, and run no servers that hold your day-count records. The app is a one-time engineering build plus ongoing rule updates — not a subscription to a SaaS pipeline. Read more on the about page.
Found an error?
If you spot a rule that's wrong, outdated, or missing, please email support@parra.io. We respond to every report and ship corrections in the next app release.