User identity and timezone

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 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 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:

// 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.