JavaScript / TypeScript

dtrexp-js is the reference implementation of DTRExp, in TypeScript. ESM, zero runtime dependencies, 100% coverage and 100% mutation score, driven by the shared conformance vectors.

Install

npm i dtrexp

Requires Node.js ≥ 20.

Quick Start

import { parse } from 'dtrexp';

const dtr = parse('T0900:1800 E1:5');

// coverage — O(#components), built for per-request hot paths
dtr.covers(new Date(), { tz: 'Europe/Berlin' });
// —> true (weekday, 09:00–18:00 Berlin local time)

// enumeration on demand — a finite window is always a finite list
dtr.intersect('2026-07-06T00:00:00Z', '2026-07-13T00:00:00Z');
// —> 5 intervals, one per business day

// "when does it next apply?"
dtr.next('2026-07-11T10:00:00Z');
// —> { start: 2026-07-13T09:00:00Z, end: 2026-07-13T18:00:00Z }

Parse once (at write/config time), evaluate many; DTRExp instances are immutable, and covers() performs a single calendar-field extraction followed by integer comparisons. No occurrence iteration, ever.

Examples

An access-control grant that only applies during business hours; the expression lives in the grant as data, and the check runs on every request:

import { parse } from 'dtrexp';

// grant.scope = "T0900:1800 E1:5"  — stored in your DB / ACL
const scope = parse(grant.scope);

function authorize(req) {
  if (!scope.covers(Date.now(), { tz: req.user.tz })) {
    throw new ForbiddenError('outside the permitted window');
  }
  // … proceed
}

A maintenance window every 10 days from an anchor date; “when is the next one?” is one call:

const win = parse('20200106/10D T0300:0500');
win.next(new Date());
// —> { start: …, end: … } — the next 03:00–05:00 slot on the 10-day cadence

Validation with the spec’s unsatisfiability lint; D30 M2 parses but can never match, and the library tells you so instead of failing silently:

import { validate } from 'dtrexp';

validate('D30 M2');
// —> { valid: true, errors: [], warnings: [{ code: 'unsatisfiable', … }] }

Interop and display:

parse('E7#-1 M4').describe();
// —> 'the last Sunday in April'

parse('D25 M12').toRRule();
// —> 'RRULE:FREQ=YEARLY;BYMONTH=12;BYMONTHDAY=25'

API

Functions

Function	Description
`parse(expression)`	Parses a DTRExp string into an immutable `DTRExp`. Throws `DTRExpSyntaxError` with a stable `code` and character `position` on invalid input. The only way to construct a `DTRExp`.
`validate(expression)`	Non-throwing variant. Returns `{ valid, errors, warnings }`; warnings include the spec’s unsatisfiability lint.

`DTRExp` Instance

Member	Description
`covers(instant, opts?)`	Whether the expression covers the instant. O(#components) integer tests after one field extraction.
`intersect(start, end, opts?)`	Covered intervals clipped to `[start, end)`: a finite, sorted, merged list of half-open `{ start: Date, end: Date }` intervals.
`next(after, opts?)`	The first maximal covered interval starting strictly after `after` (coverage containing `after` is skipped). `null` when nothing starts before the year-9999 horizon: the coverage is exhausted, or it is continuous from `after` on (`E1:7` covers every instant, so nothing ever starts); neither means “never applies”, `covers(after)` says whether it applies now.
`covering(instant, opts?)`	The maximal covered interval containing `instant`, or `null` when it is not covered (`covers(t)` holds iff `covering(t)` is not `null`). The “applies now, until …” half of a display; `next()` is the “next applies at …” half. Coverage that reaches the edge of the year 1–9999 domain starts or ends there.
`describe(locale?)`	Human-readable English rendering. v1 supports `'en'`; the parameter is reserved.
`toRRule()`	RFC 5545 RRULE (+ `DTSTART` when anchored) for the losslessly-mappable subset, else `null`. Constrained cadences emit RFC 7529 `SKIP=BACKWARD`.
`toString()`	Canonical normalized form (redundant components dropped, canonical order, wraps re-fused).
`warnings`	The §9.1 warnings of the parsed expression; same content as `validate().warnings`.
`source`	The original expression, verbatim.

Inputs & Options

Instants: Date, epoch milliseconds, ISO 8601 string, or any Temporal-like object exposing epochMilliseconds (no Temporal dependency).
opts.tz: IANA time zone for evaluation, default 'UTC'. The zone is always an evaluation parameter, never part of the expression. DST is handled per spec §9.3: spring-forward gap times cover nothing; repeated fall-back times are covered on both passes. intersect() and next() return exactly the instants covers() accepts, transition days included: a clock time inside a gap yields no interval, a repeated one yields two.

Quality

The test suite is driven by the shared vectors.json, including the calendar traps (Feb 29 in 2000/2024/2100, W53 existence, DST gap/overlap, constrain arithmetic on month-end anchors). 100% coverage on all four metrics and a 100% Stryker mutation score, both enforced in CI; inclusivity mutants (< vs <=) are exactly the class of bug a date-range library must not ship.