🕐 Timezone Hermes Fix

Timezone Hermes Fix is a cross-platform React Native library that provides a robust workaround for timezone-related issues when using the Hermes JavaScript engine. It includes native modules for both Android and iOS, designed as a drop-in solution for React Native projects requiring accurate timezone handling.

🚀 React Native Support

🔧 The Problem

React Native apps using the Hermes JavaScript engine may experience timezone calculation issues, particularly when:

• The device timezone changes while the app is running
• Date/time calculations don't reflect the current timezone
• Inconsistent behavior across different devices or regions

This library fixes these known issues by providing native timezone cache reset.

📦 Installation

Using Yarn (recommended)

yarn add @callstack/timezone-hermes-fix

Using npm

npm install @callstack/timezone-hermes-fix

🎯 Usage

Quick Start with Hook

The easiest way to use the library is with the provided React hook:

import React from 'react';
import { View, Text } from 'react-native';
import { useTimezoneHermesFix } from '@callstack/timezone-hermes-fix';

export default function App() {
  const { currentTimezone } = useTimezoneHermesFix();

  return (
    <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
      <Text>Current Timezone: {currentTimezone}</Text>
      <Text>Current Time: {new Date().toLocaleString()}</Text>
    </View>
  );
}

Direct Native Module Usage

For more control, you can use the native module directly:

import { NativeTimezoneHermesFix } from '@callstack/timezone-hermes-fix';

// Get current timezone information
const timezone = NativeTimezoneHermesFix.getCurrentTimeZone();
console.log('Timezone:', timezone.name); // "America/New_York"
console.log('GMT Offset:', timezone.secondsFromGMT); // -18000 (for EST)
console.log('DST Active:', timezone.isDaylightSavingTime); // true/false

// Get all supported timezones
const supportedTimezones = NativeTimezoneHermesFix.getSupportedTimeZones();
console.log('Available timezones:', supportedTimezones.length);

// Listen for timezone changes
const subscription = NativeTimezoneHermesFix.onTimezoneChange.addListener(
  (newTimezone) => {
    console.log('Timezone changed to:', newTimezone.name);
    console.log('New GMT offset:', newTimezone.secondsFromGMT);
    console.log('DST status:', newTimezone.isDaylightSavingTime);
  }
);

// Clean up listener when component unmounts
// subscription.remove();

📱 API Reference

Types

type CurrentTimezone = {
  name: string; // e.g., "America/New_York"
  secondsFromGMT: number; // Offset in seconds from GMT
  isDaylightSavingTime: boolean; // Whether DST is currently active
};

Methods

getCurrentTimeZone(): CurrentTimezone

Returns the current timezone information.

getSupportedTimeZones(): string[]

Returns an array of all supported timezone identifiers.

onTimezoneChange: EventEmitter<CurrentTimezone>

Event emitter that fires when the device timezone changes.

Hook

useTimezoneHermesFix(): { currentTimezone: string }

React hook that automatically updates when the timezone changes.

🧪 Example App

A complete example app is included in the repository. To run it:

# Clone the repository
git clone https://github.com/callstack/timezone-hermes-fix.git
cd timezone-hermes-fix

# Install dependencies
yarn

# Run on iOS
yarn example ios

# Run on Android
yarn example android

🤝 Contributing

We welcome contributions! Please see our Contributing Guide for details.

📄 License

This project is licensed under the MIT License. See LICENSE for details.

Made with ❤️ at Callstack

@callstack/timezone-hermes-fix is an open source project and will always remain free to use. If you think it's cool, please star it 🌟. Callstack is a group of React and React Native geeks, contact us at [email protected] if you need any help with these or just want to say hi!