Getting the client's time zone (and offset) in JavaScript

Digging into the Intl.DateTimeFormat method

The Intl.DateTimeFormat is a constructor for language-sensitive date and time formatting. Its resolvedOptions() returns an object containing properties derived from the formatting options, including the current runtime's time zone.

Decoding daylight saving time

Utilizing only the offset for time zone logic can lead to unforeseen errors due to changes in daylight saving time (DST). Offset values can change by an hour, and not all regions even observe DST. Accuracy depends heavily on the IANA time zone data for localized time displays and event scheduling.

Cross-Browser Support and Compatibility

Intl.DateTimeFormat has exceptionally high cross-device and cross-browser compatibility. However, subtle nuances exist. With a compatibility rate of >95% globally, we can expect future ECMAScript versions to simplify time zone retrieval methods.

The Nitty-Gritty of UTC/GMT offsets

Still, grasping getTimezoneOffset is beneficial. It returns the offset in minutes from UTC, with positive values indicating regions west of UTC. Unlike conventional notation (e.g., UTC-5), JavaScript works in reverse: positive numbers represent time zones behind UTC and vice versa.

Exploring JavaScript Libraries

Few problems exceed the complexity of timezone issues. JavaScript libraries such as Luxon, date-fns, and Day.js provide sophisticated IANA time zone support and manage DST transitions, besides facilitating date arithmetic and time zone conversion.

Managing edge-cases

Time zones aren't all hunky-dory, sometimes they entail half-hour and 45-minute offsets. Here's a handy piece of code to handle non-standard offsets:

let offsetMinutes = new Date().getTimezoneOffset();
// Why yes we do maths!
let hours = Math.floor(Math.abs(offsetMinutes) / 60);
let minutes = Math.abs(offsetMinutes) % 60;
// Format offset for Parse in idiosyncratic JS fashion
let formattedOffset = `${offsetMinutes > 0 ? '-' : '+'}${String(hours).padStart(2, '0')}:${String(minutes).padStart(2, '0')}`;
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Ensure proper sign representation and zero-padding for both hours and minutes.

Extract Time Zone Information from Strings

Time zone data might be nestled within a datetime string. Voila, regex can help you extract that:

let datetimeString = `2023-03-15T12:00:00-07:00`;
let timeZoneMatch = datetimeString.match(/([+-][0-9]{2}:[0-9]{2})$/);
let timeZoneString = timeZoneMatch ? timeZoneMatch[1] : '';
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The Unique Value of Time Zone Capture

Accurate time zone capture significantly impacts the accuracy of temporal data in applications. It affects content delivery, date calculations, user experiences, event scheduling, and more.

Utilizing IANA Time Zone Database

Leverage the IANA time zone database as a definitive resource for a holistic understanding and implementation of time zone support. Regularly updated, this database caters to political and socio-economic changes affecting time zones globally.