What regional defaults should you read from the Locale instead of assuming: units, 12 or 24 hours, ordinals and date order?

24.9.3 · L3 · Interview

Predict first

What do locales, date styles, units and ordinals print?

import Foundation

for id in ["en_US", "en_GB", "de_DE", "ja_JP", "ar_EG", "th_TH"] {
    let l = Locale(identifier: id)
    print(id, l.measurementSystem.identifier, l.hourCycle, l.currency?.identifier ?? "-", l.numberingSystem.identifier, l.calendar.identifier)
}
let date = Date(timeIntervalSince1970: 1_700_000_000)
for id in ["en_US", "en_GB", "de_DE", "ja_JP"] {
    var time = Date.FormatStyle(date: .omitted, time: .shortened).locale(Locale(identifier: id)); time.timeZone = TimeZone(identifier: "UTC")!
    var day = Date.FormatStyle().year().month(.abbreviated).day().locale(Locale(identifier: id)); day.timeZone = TimeZone(identifier: "UTC")!
    print(id, date.formatted(time), "|", date.formatted(day))
}
let temp = Measurement(value: 20, unit: UnitTemperature.celsius)
let mf = MeasurementFormatter(); mf.unitOptions = .naturalScale
mf.locale = Locale(identifier: "en_US"); print(mf.string(from: temp))
mf.locale = Locale(identifier: "de_DE"); print(mf.string(from: temp))
mf.unitOptions = .providedUnit; mf.locale = Locale(identifier: "en_US"); print(mf.string(from: temp))
let n = NumberFormatter(); n.locale = Locale(identifier: "en_US")
n.numberStyle = .ordinal; print([1, 2, 3, 11, 22].compactMap { n.string(from: NSNumber(value: $0)) })
n.locale = Locale(identifier: "fr_FR"); print([1, 2].compactMap { n.string(from: NSNumber(value: $0)) })

en_US ussystem oneToTwelve USD latn gregorian / … / 68°F / 20 °C / 20°C / ["1st", "2nd", "3rd", "11th", "22nd"] / ["1er", "2e"]
All locales print metric
20°C for every locale

Mental model. Asking a shop for “a large coffee” means different volumes in different countries. The locale is the shop’s menu: you ask for the item and it knows the local size.

The Locale carries decisions you would otherwise guess wrong. The check lists them for six locales: the measurement system (United States uses its own system, Great Britain its mixed UK one, Germany, Japan, Egypt and Thailand metric), the hour cycle (English-US and Egyptian Arabic use 1 to 12, the others 0 to 23), the currency (USD, GBP, EUR, JPY, EGP, THB), the numbering system (Egyptian Arabic uses Arabic-Indic digits) and the calendar (Thailand uses the Buddhist one by default). Formatters apply them for you: the same instant prints “10:13 PM” and “Nov 14, 2023” in en_US, “22:13” and “14 Nov 2023” in en_GB, “14. Nov. 2023” in de_DE and “2023年11月14日” in ja_JP, so never hand-write “MM/dd/yyyy” for display; use FormatStyle with the components you want (Q83) and let the locale arrange them. Note that the user can override the hour cycle in Settings regardless of language, which a fixed format string ignores.

Units are the classic trap. A MeasurementFormatter with .naturalScale shows 20 degrees Celsius as “68°F” for en_US and “20 °C” for de_DE, because it converts to the unit the region uses; .providedUnit keeps Celsius for everyone. Choose deliberately: use natural scale for display to people, provided unit for scientific or fixed contexts, and store canonical units (SI) in your model. Ordinals (1st, 2nd, 3rd, 11th) are rule-based and language-specific (“1er” and “2e” in French), so use NumberFormatter with .ordinal rather than a suffix table, and keep sentences that include them in the catalog with a comment (Q110). Do not infer region from language (a user with an English language and a German region is normal, Q82), read the full structure from Locale.current or autoupdatingCurrent, and pin the locale in tests (Q447). Output was checked on macOS, not on a device with the user’s own overrides.

Regional defaults and natural-scale units come from the locale.

Common traps

Using one fixed date or unit format for display. It is right in one region only.
Converting units yourself with an “is metric” flag. The system already knows the region’s system and rounding.

They might follow up with

When do I want .providedUnit? When the unit is part of the content (a medical dose, a scientific reading).
How do I test other regions? Scheme App Region, or a formatter with an explicit locale as the check does.

What regional defaults should you read from the Locale instead of assuming: units, 12 or 24 hours, ordinals and date order?

Predict first

Common traps

They might follow up with

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