import { Temporal } from "@js-temporal/polyfill";
import type { DateTimeDurationUnit, DurationStringOptions, RoundingOptions } from "../../types/index.js";
/**
 * Return the difference between two UTC datetimes as an ISO 8601 duration string,
 * bridging to the `duration` namespace (see `parseDuration`, `normalizeDuration`).
 *
 * - Uses Temporal.Instant.until() with `largestUnit` set to `unit`, then `.toString()`.
 * - Unlike `diffUtc`, `unit` is a single unit (not an array) — an ISO duration string
 *   already expresses a full multi-unit breakdown via `largestUnit` alone, so there's no
 *   array-of-units overload here.
 * - Returns `""` for invalid input (negative diffs are valid and render with a leading `-`).
 *
 * `smallestUnit`, `roundingIncrement`, and `roundingMode` control optional rounding of the
 * underlying difference before it's rendered, per Temporal's DifferenceOptions — same as
 * `diffUtc`. `toStringSmallestUnit`, `fractionalSecondDigits`, and `toStringRoundingMode`
 * control the precision of the rendered string itself, per Temporal's ToStringPrecisionOptions
 * (mirroring `parseDuration`'s options) — kept separate from the `.until()` rounding options
 * above because both option sets have colliding `smallestUnit`/`roundingMode` keys with
 * different Temporal types.
 *
 * @param value1 UTC ISO datetime string (start)
 * @param value2 UTC ISO datetime string (end)
 * @param unit DateTimeDurationUnit to use as the duration's largestUnit
 * @param options optional: smallestUnit, roundingIncrement, roundingMode (.until() rounding); toStringSmallestUnit, fractionalSecondDigits, toStringRoundingMode (.toString() precision)
 * @returns ISO 8601 duration string, or "" on invalid input
 *
 * @example diffUtcAsDuration("2024-03-10T12:00:00Z", "2024-03-11T12:00:00Z", "hours") // "PT24H"
 * @example diffUtcAsDuration("2024-03-11T12:00:00Z", "2024-03-10T12:00:00Z", "hours") // "-PT24H"
 * @example diffUtcAsDuration("invalid", "2024-03-11T12:00:00Z", "hours") // ""
 */
export declare function diffUtcAsDuration(value1: string, value2: string, unit: DateTimeDurationUnit, options?: RoundingOptions<Temporal.DateTimeUnit> & DurationStringOptions): string;