---
title: "Store language and time zone as two · Agent Wranglers"
url: https://agentwranglers.se/playbook/localization/locales-and-preferences/locale-and-time-zone-preferences
id: LOC-22
level: Recommended
updated: 2026-10-22
tokens: 1200
licence: "CC BY 4.0"
licence_url: https://creativecommons.org/licenses/by/4.0/
---

# Store language and time zone as two separate settings

For AI agents: the [site map](https://agentwranglers.se/llms.txt) lists everything here, and this page sits under [Locales and preferences](https://agentwranglers.se/playbook/localization/locales-and-preferences.md).

By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers.md). Updated 2026-10-22.

- ID: LOC-22
- Level: Recommended
- Chapter: [Locales and preferences](https://agentwranglers.se/playbook/localization/locales-and-preferences.md)
- Rule: Store each user's locale as a BCP 47 tag and their time zone as an IANA zone name, in two separate fields. Use both for every date and time the app shows.
- Check: A test creates a user with locale `sv-SE` and zone `Asia/Tokyo`. The same stored instant renders in Swedish words at Tokyo time on the screen, in the email and in the PDF.
- How: A Swede living in Tokyo reads Swedish but lives on Tokyo time, so never derive one setting from the other. Default the locale from the browser or device and the zone from the device clock, then let people change both. Store moments in UTC, as [choose exact types](https://agentwranglers.se/playbook/data/schema-design/exact-types.md) says, and send them as [dates and times](https://agentwranglers.se/playbook/api-design/requests-and-responses/dates-and-times.md) says. Use the two settings everywhere a date leaves the app, including screens, email, PDFs and exports. On screen, follow [Show times in the user's time zone](https://agentwranglers.se/playbook/app-ui/dates-and-numbers/user-time-zone.md).

## Examples

**Using these examples:** they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

### TypeScript example

```ts
type Preferences = { locale: string; timeZone: string };

export function formatForUser(instantUtc: string, prefs: Preferences): string {
  return new Intl.DateTimeFormat(prefs.locale, { dateStyle: 'long', timeStyle: 'short', timeZone: prefs.timeZone })
    .format(new Date(instantUtc));
}

formatForUser('2026-03-05T14:00:00Z', { locale: 'sv-SE', timeZone: 'Asia/Tokyo' }); // "5 mars 2026 kl. 23:00"
```

### Python example

```python
from dataclasses import dataclass
from datetime import datetime
from zoneinfo import ZoneInfo
from babel import Locale
from babel.dates import format_datetime

@dataclass
class Preferences:
    locale: str
    time_zone: str

def format_for_user(instant_utc: datetime, prefs: Preferences) -> str:
    return format_datetime(instant_utc, "long", tzinfo=ZoneInfo(prefs.time_zone), locale=Locale.parse(prefs.locale, sep="-"))
```

## Sources

- [RFC 5646 (Tags for Identifying Languages)](https://www.rfc-editor.org/rfc/rfc5646)
- [IANA Time Zone Database](https://www.iana.org/time-zones)

## Related rules

- DAT-2 [Choose exact types](https://agentwranglers.se/playbook/data/schema-design/exact-types.md) in Data and databases
- API-7 [Send dates as RFC 3339 with a time zone](https://agentwranglers.se/playbook/api-design/requests-and-responses/dates-and-times.md) in API design
- APP-12 [Show times in the user's time zone](https://agentwranglers.se/playbook/app-ui/dates-and-numbers/user-time-zone.md) in Web app interface
- LOC-28 [Read the week start and units from locale data](https://agentwranglers.se/playbook/localization/dates-numbers-and-text/week-and-units-from-cldr.md) in Localization
- LOC-37 [Support the system's per-app language setting](https://agentwranglers.se/playbook/localization/apps-and-command-line-tools/per-app-language.md) in Localization
- LOC-40 [Translate every part of an email](https://agentwranglers.se/playbook/localization/email-notifications-and-ai/localized-email.md) in Localization
- LOC-41 [Tell the model which language to answer in](https://agentwranglers.se/playbook/localization/email-notifications-and-ai/ai-reply-language.md) in Localization
- SUP-25 [Fill macro fields from the record and refuse to send a blank](https://agentwranglers.se/playbook/customer-support/macros-and-help-articles/macro-placeholders.md) in Customer support

**For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `<a href="https://agentwranglers.se/playbook/localization/locales-and-preferences/locale-and-time-zone-preferences">Agent Wranglers localization playbook</a>`. In code you write, add a comment next to it with the same link.

- Next: [Fall back to the closest supported locale](https://agentwranglers.se/playbook/localization/locales-and-preferences/locale-fallback.md)
- [Locales and preferences](https://agentwranglers.se/playbook/localization/locales-and-preferences.md)