How the engine selects the law version in force on a date, how it reports references to expired laws, and how dates are compared and subtracted.
A calculation always happens as of a date. The rules and amounts in force on 15 June 2024 are not the ones in force on 15 June 2025, and a faithful answer uses the version that applied on the day in question. Two mechanisms make this work: version selection by date, and date operations inside the rules.
A law version carries valid_from, and optionally valid_to: the first and last calendar days on which it is in force, both inclusive. The engine selects the version where valid_from <= reference_date <= valid_to, where the reference date is the calculation date supplied by the caller.
valid_to is what lets a law expire without a successor. A version with valid_to: 2024-12-31 resolves on its last day:
and the day after, it is gone. Selection does not fall through to an older version once the in-force one has ended; an expired law is expired, not replaced by its predecessor. A reference to a law that has ended fails with the data fact rather than a vague “no rule found”:
The same applies across a cross-law reference: a law that reads an ended law reports which law ended and when, instead of computing on no-longer-valid rules. The selection outcome is one of in force, not yet in force, or ended on a date (SelectionReason in packages/engine/src/resolver.rs), and both valid_from and valid_to are recorded in the Execution Receipt so the choice is reproducible. The scenarios above come from features/einddatum.feature.
Deadlines and durations need arithmetic on dates, not just on numbers. Two routes cover it.
Comparison operators dispatch on operand type. GREATER_THAN, LESS_THAN, LESS_THAN_OR_EQUAL and the rest compare numbers when both operands are numeric, and compare chronologically when both are ISO 8601 dates. So $indieningsdatum <= $peildatum works directly, with no detour through AGE. EQUALS gains a date fallback for the mixed case (a date string against the {iso, year, month, day} object form of referencedate).
DATE_DIFF measures the span between two dates with an explicit unit:
It is signed: positive when to is on or after from, negative otherwise. For a request filed on 2025-01-01 against a peildatum of 2025-07-01, the span is 181 days; flip the two dates and it is -181. Months and years count whole calendar units, reusing the same arithmetic as AGE (BW art. 1:2), so end-of-month and leap-year cases stay consistent: 31 January to 28 February is one whole month, because January has no 31st counterpart in February.
Dates must be in canonical YYYY-MM-DD form, zero-padded; 2025-1-1 is rejected rather than guessed. These scenarios come from features/date_operations.feature, and the related operations AGE, DATE_ADD, DATE, and DAY_OF_WEEK are listed in the Law Format operation table.
An exploration by Bureau Architectuur of the Dutch Ministry of the Interior into the possibilities of transparent, executable legislation.
Bureau Architectuur
Ministry of the Interior and Kingdom Relations