Use GET on /account/:id/interest with an interestMonth query value in YYYY-MM format to retrieve interest reported by the account's provider for a calendar month. Future calendar months are rejected. The interest payout is the rounded amount reported by the provider, including carryover, already paid or payable for the period. APYE applies to the returned period, while interest YTD reflects the calendar year current when the observation is made, including for historical requests. Periods may be partial, and observedAt records successful retrieval and validation. Interest is omitted when it is unavailable for the account.