Catch timezone bugs with ESLint

Keep new Cloudflare Workers code on the same timezone policy. Pair @rdlabo/workers-timezone with @rdlabo/eslint-plugin-rules to catch host-local Date / Intl operations and request-scoped initialization.

For a runnable demo of both packages, start with Try conversions and lint.

Enable the companion preset

This example uses Node.js 24. In an app using @rdlabo/workers-timezone, install the lint dependencies:

npm install --save-dev eslint@10 @eslint/js@10 typescript@6 typescript-eslint@8 @rdlabo/eslint-plugin-rules@22

Add the timezone preset and typed linting to eslint.config.mjs. The TypeScript files you lint must be included in the app's tsconfig.json.

import { dirname } from 'node:path';
import { fileURLToPath } from 'node:url';
import eslint from '@eslint/js';
import tseslint from 'typescript-eslint';
import rdlabo from '@rdlabo/eslint-plugin-rules/typescript';

export default tseslint.config(
  eslint.configs.recommended,
  {
    files: ['**/*.ts'],
    extends: [...tseslint.configs.recommendedTypeChecked],
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: dirname(fileURLToPath(import.meta.url)),
      },
    },
    plugins: { '@rdlabo/rules': rdlabo },
  },
  ...rdlabo.configs['workers-timezone/recommended'],
);

Keep your existing configuration entries when merging this setup. The separate workers/recommended preset does not enable timezone checks.

Catch a host-local date

This reads the host's calendar date instead of your application's:

const instant = new Date('2026-01-01T15:00:00Z');
console.log(instant.getDate());

Use the library to select the calendar timezone:

import { toLocalDate } from '@rdlabo/workers-timezone';

const instant = new Date('2026-01-01T15:00:00Z');
console.log(toLocalDate(instant, 'Asia/Tokyo'));
// 2026-01-02

no-implicit-timezone also reports supported Intl formatting calls without an explicit timeZone. UTC methods and toISOString() remain available for instant-based operations.

Keep initialization out of requests

initialize-timezone-at-module-scope reports this:

import { initializeTimezone } from '@rdlabo/workers-timezone';

export function handleRequest() {
  initializeTimezone({ timeZone: 'Asia/Tokyo' });
}

Initialize during module evaluation instead:

import { initializeTimezone } from '@rdlabo/workers-timezone';

initializeTimezone({ timeZone: 'Asia/Tokyo' });

For a user's timezone, pass an explicit argument to the conversion. The initialization rule checks each file; it does not enforce a single initialization across the entire app.

Run it in CI

After installing dependencies, run:

npx eslint 'src/**/*.ts' --max-warnings 0

Adjust the source path for your app. Keep timezone tests for DST and calendar boundaries: static checks cover recognizable operations, not every dynamic value.

See the Date / Intl rule, initialization rule, and timezone behavior.