DO NOT USE IN PRODUCTION. It can only use for tests.

README.md

import assert from './assert'
import { runInTimezone } from './runInTimezone'

const offset = await runInTimezone('Asia/Tokyo', () => new Date().getTimezoneOffset())
assert.strictEquals(offset, -540)

eval-call.js

if (typeof process.env.RUN_IN_TIMEZONE_CODE !== 'string') {
  throw new Error('process.env.RUN_IN_TIMEZONE_CODE must be string')
}

// eslint-disable-next-line no-eval
const fn = eval(`(${process.env.RUN_IN_TIMEZONE_CODE})`)
if (typeof fn !== 'function') {
  throw new Error('process.env.RUN_IN_TIMEZONE_CODE must be string of Function')
}

// eslint-disable-next-line no-undef
process.send(fn())

runInTimezone.js

export const runInTimezone = (timezone: string, fn: Function): Promise<any> => {
  const file = path.join(__dirname, 'eval-call.js')
  return new Promise((resolve, reject) => {
    const subprocess = fork(file, [], {
      env: {
        ...process.env,
        TZ: timezone,
        RUN_IN_TIMEZONE_CODE: fn.toString(),
      },
    })
    subprocess.on('message', resolve)
    subprocess.on('error', reject)
  })
}