/**
 * Computes an updated system clock offset from a server Date header.
 *
 * When `timeRequestSent` is provided, uses the midpoint formula:
 *   elapsed        = timeResponseReceived - timeRequestSent
 *   midpoint       = (timeRequestSent + timeResponseReceived) / 2
 *   candidateSkew  = serverTime - midpoint
 *
 * When `timeRequestSent` is absent (legacy callers), falls back to:
 *   candidateSkew  = serverTime - timeResponseReceived
 *
 * The candidate is discarded if:
 *   - An Age header is present
 *   - elapsed > 15 minutes
 *
 * The candidate is recorded unconditionally when not
 * discarded. The detection threshold (4 min) is only used for retry decisions,
 * not for whether to update the offset.
 *
 * @internal
 * @param clockTime The string value of the Date response header.
 * @param currentSystemClockOffset The current system clock offset in milliseconds.
 * @param timeRequestSent The raw client time (ms) at which the request was sent.
 * @param ageHeader The value of the Age response header, if present.
 */
export declare const getUpdatedSystemClockOffset: (clockTime: string, currentSystemClockOffset: number, timeRequestSent?: number, ageHeader?: string) => number;