Biome lint rule gap for regional time zone conversion logic

There is no confirmed public commitment that Biome will add lint rules that detect unsafe regional time zone conversions. The likely explanation is architectural: Biome is a static analyzer and formatter with no date-time runtime, so it cannot evaluate behavior that depends on the host machine's locale or zone. Temporal correctness is therefore expected to stay largely outside the linter's core scope, delegated to libraries and tests. Treat this as "not planned" rather than "never," and confirm against your installed version's rule list and release notes, since rule sets shift between minor releases.

Why static linting fits this problem poorly

Biome's rules are largely a curated subset of ESLint-style checks targeting syntax, style, and obvious API misuse. Judging whether a conversion is "unsafe" requires knowing the runtime zone, the input format, and the semantics of whichever date library is in use (Luxon, Day.js, or Temporal). A linter can only approximate this by banning API shapes, not by evaluating correctness. A clean lint run is never evidence of time-zone-safe code.

What you can enforce today

• Standardize on one date library with explicit zone arguments, and enforce it through code review conventions.
• Write custom GritQL plugin rules (supported in recent Biome versions; verify yours) to flag patterns like Date.parse("10/14/2026"), new Date() with non-ISO strings, or Intl.DateTimeFormat calls that omit an explicit timeZone option.
• Run tests under multiple zones by setting TZ=UTC and a non-UTC zone in CI. This catches host-dependent parsing that no linter can see.

Verifying the gap yourself

Create a fixture and lint it to see what your version actually catches:

const a = new Date("10/14/2026");
const b = Date.parse("2026-10-14 20:35");
const c = new Date().toLocaleString();
const z = new Intl.DateTimeFormat().resolvedOptions().timeZone;

If no diagnostics fire, that confirms the boundary: Biome stays agnostic, and zone safety is your test suite's job. The Temporal proposal may eventually give linters zone-aware types to reason about, but adoption is uneven, so don't plan around it.