/**
 * Parse an RFC 7231 IMF-fixdate string — the fixed grammar HTTP headers like
 * `Last-Modified`/`Date`/`Expires` use — into a UTC ISO 8601 datetime string.
 *
 * - **Decoding, not display.** Accepts English weekday/month abbreviations
 *   only, per the fixed grammar (see `formatHttp`'s JSDoc and roadmap
 *   Decision 1).
 * - **IMF-fixdate only.** RFC 7231 also lists two obsolete forms
 *   (`rfc850-date`, `asctime-date`) that real HTTP servers occasionally
 *   still emit; this is a documented limitation — neither is accepted here.
 * - Day, hour, minute, and second must each be exactly 2 digits and the
 *   trailing zone must be the literal `GMT` — unlike `parseRfc2822`, no
 *   numeric offset or named-zone table is accepted.
 * - The day-of-week is not cross-validated against the computed date, the
 *   same deliberate scope limit `parseDateTimeWithPattern`/J11 uses.
 *
 * @param value RFC 7231 IMF-fixdate string (e.g. "Fri, 15 Mar 2024 14:30:00 GMT")
 * @returns UTC ISO 8601 datetime string, or "" on invalid input
 *
 * @example parseHttp("Fri, 15 Mar 2024 14:30:00 GMT") // "2024-03-15T14:30:00Z"
 * @example parseHttp("Fri, 15 Mar 2024 14:30:00 -0400") // "" (not IMF-fixdate)
 * @example parseHttp("not a date") // ""
 */
export declare function parseHttp(value: string): string;