Format every date and amount from the reader's locale and zone

How to do it

Format with an ICU-based library, such as Intl in JavaScript, Babel in Python, the intl extension in PHP or ICU4J in Java. Always pass a locale, and pass a time zone for dates and times. Store and send moments in UTC, as in dates and times, and convert only for display. Take the zone from the person's setting, as Store language and time zone as two separate settings says.

Examples

JavaScript example

const when = new Date('2026-03-05T14:00:00Z');
new Intl.DateTimeFormat('sv-SE', { dateStyle: 'long', timeStyle: 'long', timeZone: 'Europe/Stockholm' }).format(when);
// "5 mars 2026 kl. 15:00:00 CET"
new Intl.NumberFormat('de-DE', { style: 'currency', currency: 'EUR' }).format(1234.5);
// "1.234,50 €", with a no-break space before the €

Python example

# Python with Babel
from datetime import datetime, timezone
from zoneinfo import ZoneInfo
from babel.dates import format_datetime
from babel.numbers import format_currency
when = datetime(2026, 3, 5, 14, 0, tzinfo=timezone.utc)
format_datetime(when, "long", tzinfo=ZoneInfo("Europe/Stockholm"), locale="sv_SE")
# "5 mars 2026 15:00:00 CET"
format_currency(1234.5, "EUR", locale="de_DE")  # "1.234,50 €"

PHP example

// PHP with the intl extension
$when = new DateTimeImmutable('2026-03-05T14:00:00Z');
$dates = new IntlDateFormatter('sv_SE', IntlDateFormatter::LONG, IntlDateFormatter::LONG, 'Europe/Stockholm');
echo $dates->format($when); // "5 mars 2026 kl. 15:00:00 CET"
echo (new NumberFormatter('de_DE', NumberFormatter::CURRENCY))->formatCurrency(1234.5, 'EUR'); // "1.234,50 €"

How to check

A unit test formats a fixed date and amount for each supported locale. The output matches the expected strings.

Sources

• ECMA-402 (ECMAScript Internationalization API)
• Unicode CLDR

Related rules

• APP-12 Show times in the user's time zone in Web app interface
• API-7 Send dates as RFC 3339 with a time zone in API design
• LOC-22 Store language and time zone as two separate settings in Localization
• LOC-26 Writing date formats by hand in Localization
• LOC-32 Send raw values with standard codes instead of formatted text in Localization