> For the complete documentation index, see [llms.txt](https://docs.january.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.january.ai/client-sdks/react-native-sdk/concepts/user-identity-and-timezone.md).

# User identity and timezone

```ts
const january = new JanuaryClient({
  endUserId: user.id,
  timezone: Intl.DateTimeFormat().resolvedOptions().timeZone,
  clientTokenProvider: tokenProvider,
});
```

## End-user ID

`endUserId` is required. It is the stable, opaque [end-user ID](/docs/authentication.md) your backend binds into the client token: your own account ID, never an email address or another value that can change.

The token decides whose logs are read and written. The SDK passes `endUserId` to your token provider, and sends it to January as the `January-End-User-ID` header only in `developmentApiKey` mode.

## Timezone

Pass the device's IANA timezone, as above, unless you store one per user. It sets the calendar days for food-log lists and summaries and for water and weight lists, and it is sent with glucose predictions. [Days and timezones](/rest-api/api-overview.md#days-and-timezones) explains how January groups logs into days.

If you omit it or pass an invalid name, the result depends on the platform:

|         | Omitted             | Invalid name                                                           |
| ------- | ------------------- | ---------------------------------------------------------------------- |
| iOS     | The device timezone | `new JanuaryClient` throws `timezone must be a valid IANA identifier.` |
| Android | UTC                 | Accepted; calls that send it fail with `invalid_request`               |

Build `YYYY-MM-DD` dates in the same timezone. `new Date().toISOString().slice(0, 10)` is the UTC date, which in the Americas is already tomorrow by the evening:

```ts
// The device's calendar date as YYYY-MM-DD.
const localDate = (d = new Date()) =>
  `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`;
```

## Log times

`timestampUTC` (food logs), `consumedAt` (water) and `measuredAt` (weight) are the REST API's `created_at`: when the meal was eaten, the water drunk, or the weight measured. Send ISO 8601 with an offset, or omit it to mean now. Responses return it in UTC.