/**
 * Parses an ISO 8601 duration into a number of milliseconds.
 *
 * The grammar is `PnDTnHnMnS`, where days count as exactly 24 hours, a sign is allowed both on the whole
 * duration and on each component, and the seconds may carry up to nine fractional digits. Weeks, months and
 * years are rejected: they have no fixed length, and the API models them as calendar periods rather than as
 * durations.
 *
 * @param duration - The ISO 8601 duration to parse
 * @returns The duration in milliseconds
 * @throws {Error} If the input cannot be parsed as an ISO 8601 duration
 *
 * @example
 * parseDuration('PT1H')        // 3600000
 * parseDuration('P1DT2H30M')   // 95400000
 * parseDuration('PT0.5S')      // 500
 * parseDuration('-PT1H')       // -3600000
 */
export declare function parseDuration(duration: string): number;
/**
 * Normalizes a duration into the ISO 8601 form the API expects on the wire.
 *
 * This is the duration counterpart of `normalizeDate`: it lets a duration input accept a plain number of
 * milliseconds, the way date inputs accept a timestamp. A string is assumed to be an ISO 8601 duration
 * already and is passed through untouched, because the endpoints do not all read the same grammar — some
 * take a time span (`PT1H`), others also accept a calendar period (`P1M`) — and rewriting the caller's value
 * could turn one the API accepts into one it rejects.
 *
 * @param duration - The duration to normalize, either a number of milliseconds or an ISO 8601 duration
 * @returns The duration as an ISO 8601 string, or undefined if input is null/undefined
 * @throws {Error} If the input is a number that cannot be turned into a duration
 *
 * @example
 * normalizeDuration(3600000)   // 'PT1H'
 * normalizeDuration(90500)     // 'PT1M30.5S'
 * normalizeDuration(0)         // 'PT0S'
 * normalizeDuration('P1M')     // 'P1M'
 */
export declare function normalizeDuration(duration: number | string | null | undefined): string | undefined;
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