Set the default time zone in Node.js (#tilPost)

I've been dabbling with end-to-end testing time zones the other day and discovered that setting a browser time zone is reasonably straightforward in Playwright. Setting a browser time zone wasn't enough, though.

Testing my time zone aware UI also required setting a time zone in the running Node.js process to write correct assertions. How can you set time zones in Node.js?

By default, Node.js reuses the time zone set in your operating system. To access the default time zone, use JavaScript's Intl object.

const date = new Date();
console.log(Intl.DateTimeFormat().resolvedOptions().timeZone); 
// 'Europe/Berlin'
console.log(date.toString());
// Sun Oct 20 2024 11:50:24 GMT+0200 (Central European Summer Time)

To change the used Node.js time zone, define the TZ environment variable.

// Define `TZ` either when running the Node.js script
// -> $ TZ='America/New_York' node time-zone.js
// 
// Or by setting `process.env`
// -> process.env.TZ = 'America/New_York';

const date = new Date();
console.log(Intl.DateTimeFormat().resolvedOptions().timeZone); 
// 'America/New_York'
console.log(date.toString()); 
// Sun Oct 20 2024 05:50:24 GMT-0400 (Eastern Daylight Time)

It's pretty cool that TZ is evaluated at run-time so that you can mix and match time zones in your scripts.

const date = new Date();

// Set time zone to the Americas
process.env.TZ = 'America/New_York`;
console.log(date.toString());
// Sun Oct 20 2024 05:56:40 GMT-0400 (Eastern Daylight Time);

// Set time zone to Australia
process.env.TZ = 'Australia/Sydney';
console.log(date.toString());
// Sun Oct 20 2024 20:56:40 GMT+1100 (Australian Eastern Daylight Time);

// Delete time zone and fall back to Berlin, Germany
delete process.env.TZ;
console.log(date.toString());
// Sun Oct 20 2024 11:56:40 GMT+0200 (Central European Summer Time);

And if you don't want to set a default Node.js time zone but just display dates in different time zones, there are also multiple options to only change the date formating.

const date = new Date();

console.log(date.toLocaleString()); 
// 10/20/2024, 12:06:29 PM
console.log(date.toLocaleString('en-US', { timeZone: 'Australia/Sydney' })); 
// 10/20/2024, 9:06:29 PM

console.log(
  new Intl.DateTimeFormat('en-US', {
    dateStyle: 'medium',
    timeStyle: 'long',
    timeZone: 'Australia/Sydney',
  }).format(date),
);  
// 20 Oct 2024, 21:06:29 GMT+11

I'm such an Intl fan boy... Anyways — happy time traveling!