/**
 * Formats a JS Date as an ISO 8601 string carrying its actual Europe/Berlin
 * UTC offset (e.g. "2026-06-15T09:00:00+02:00" in summer,
 * "...+01:00" in winter) — what schema.org's Event startDate/endDate
 * expect. Uses Intl's timezone database rather than hardcoded +1/+2 math,
 * so it's correct across the DST transition automatically.
 */
export function toBerlinOffsetISOString(date: Date): string {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: "Europe/Berlin",
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    hourCycle: "h23",
    timeZoneName: "shortOffset",
  }).formatToParts(date);

  const get = (type: string) => parts.find((p) => p.type === type)?.value ?? "00";
  const tzName = parts.find((p) => p.type === "timeZoneName")?.value ?? "GMT+0";
  const match = /GMT([+-])(\d+)(?::(\d+))?/.exec(tzName);
  const sign = match?.[1] ?? "+";
  const offsetHours = (match?.[2] ?? "0").padStart(2, "0");
  const offsetMinutes = (match?.[3] ?? "0").padStart(2, "0");

  return `${get("year")}-${get("month")}-${get("day")}T${get("hour")}:${get("minute")}:${get("second")}${sign}${offsetHours}:${offsetMinutes}`;
}