How every rule is built, checked and signed off

Law does not read itself. The rules are built and overseen by tax professionals — Daysium’s thirteen founding partners, the tax advisors, accountants, tax lawyers and former HMRC inspectors who advise and defend these cases, alongside a senior advisor.

Paul Aplin OBE is our senior advisor: President of the Chartered Institute of Taxation, and the first person to have led both the CIOT and the ICAEW. He is also an independent advisor to HMRC.

Among the founding partners, Alan Kennedy qualified as a senior Inspector of Taxes at HMRC and worked as a specialist investigator, then spent eight years at KPMG before founding Trident Tax, where he is Managing Director — his practice is residence, domicile and HMRC investigations. Sarah Scala directs NHD Tax Solutions and is founder and chair of The Contentious Tax Group — her view on what this record means for reasonable care closes this page.

Every rule follows the same route, and none of it is optional.

A rule is read out of the legislation itself, in the original language where that is the authority, and cited to the provision it came from. Those citations are then checked back against the source register — not a secondary summary — to confirm that the provision exists, is in force, and says what the rule claims it says. The rule is then given to a second reviewer whose task is to find the first one wrong: the thresholds, the direction, the counting basis, the window, and whether any day-type exclusion has a legal basis at all. Findings are decided one at a time and recorded with a reason, including the ones that are rejected. Only then does a named person sign the rule off.

Every rule carries the record of all of it — where it came from, who checked it, what they found, who signed it, and when it was last verified against the source.

Where the machines help, and where they stop

We use AI to do the reading: to find the legislation, work through it, pull the citations and test them back against the register.

It is not what makes the rule trustworthy. Every output goes to a person, who checks it against the source and either agrees the rule matches the law or sends it back. Nothing reaches the platform because software concluded something. A named person signs each rule off, and the record says who.

How much weight that check needs depends on the law behind it. Some jurisdictions write residence rules meant to be read by a person — a clear threshold, a defined year, nothing ambiguous enough to argue about. Those we read, verify and sign off in-house, and a tax professional would add nothing to them.

The UK is the other extreme. The Statutory Residence Test is among the most complex residence tests anywhere: tied tests, split years, deeming rules, and definitions that turn on facts nobody records by accident. When a rule reaches ground like that, it goes to our founding partners — practising tax professionals who advise on residence for a living — and they take the view. That escalation is real, and it is reserved for the rules that need it.

Where the law is unclear, we decide whether to decide

Tax law has grey areas — more of them than most people expect, and the UK has more than most. When a rule reaches one, the first question is not what position to take. It is whether to take one at all.

Sometimes we do, and we say plainly that we have: France has no day count anywhere in its tax law, so the number we count to is a judgement we have made, recorded as a judgement and never presented as statute. Sometimes we deliberately do not. We take no view on how many hours you worked on a given day in the UK — that is yours to declare, and the platform is built to let you flag it rather than to decide it for you.

Where we want a view, we have a network of tax advisors to ask. And where doubt remains, we count to the safe side. Ireland’s two-year test needs a calculation the platform does not yet make, so we hold you to a tighter single-year ceiling than the law requires and tell you that is what we have done. Every rule carries a label saying whether its number comes from legislation, from settled practice, or from a considered position of our own.

Conservative by default, and specific about which is which. It is the least exciting way to build this, and the only one that survives being read closely.

Being right once is not enough. Residence law moves — thresholds change, definitions are amended, reforms land — and a rule that matched the law when it was written stops matching it without anything happening to the rule at all.

So every ruleset names the specific legal sources it depends on, and those sources are re-checked on a schedule. Each one also carries the date it was last verified against its source, so the age of the check is visible rather than assumed.

We also read what the profession is publishing. The major firms’ residence commentary is monitored for movement — as corroboration, never as authority — and the jurisdictions where reform looks likely are kept on a watchlist, so a change is something we are expecting rather than something we discover.

Daysium covers 20+ tax jurisdictions — each one listed with the legislation it is built from, at tax jurisdictions.

Rules are half of a defensible position; the record is the other half. Daysium Mobile builds that record while the travel is happening — location and days logged on the day, not reconstructed from diaries and boarding passes months later.

That is what gives the record its weight. A contemporaneous record resists the challenge an inspector reaches for first: that the account was assembled after the event. A reconstruction invites that challenge; a record built day by day does not.

Daysium does not advise, and it does not decide. It produces the factual record and the count against the rules. Which tests apply, and what the position is, is a judgement for the taxpayer and their advisor — not for a platform.

That division is deliberate. Advice is strongest when the facts beneath it hold, and weakest when the record-keeping lets it down. Daysium exists to fix the second problem, not to enter the first.

Reasonable care is not Daysium’s claim to make. It is an argument a specialist puts to HMRC, and the specialist best placed to weigh it is one who defends these disputes for a living. Sarah Scala does — and has put her view on the record:

Her words, not ours.