export function toIso8601Full(date: Date): string {
  const yearN = date.getFullYear();
  const isNegativeYear = yearN <= 0;
  const year = isNegativeYear ? pad(1 - yearN, 6) : pad(yearN, 6);
  const signedYear = (isNegativeYear ? "-" : "+") + year;

  const month = pad(date.getMonth() + 1);
  const day = pad(date.getDate());
  const hour = pad(date.getHours());
  const minute = pad(date.getMinutes());
  const second = pad(date.getSeconds());
  const ms = pad(date.getMilliseconds(), 3);

  const dateString =
    `${signedYear}-${month}-${day}T${hour}:${minute}:${second}.${ms}`;
  const tzOffset = -date.getTimezoneOffset();
  if (tzOffset === 0) {
    return `${dateString}Z`;
  } else {
    const offsetSign = tzOffset > 0 ? "+" : "-";
    const offsetHours = pad(Math.floor(Math.abs(tzOffset) / 60));
    const offsetMinutes = pad(Math.abs(tzOffset) % 60);
    return `${dateString}${offsetSign}${offsetHours}:${offsetMinutes}`;
  }
}

export function toIso8601FullUTC(date: Date): string {
  const yearN = date.getUTCFullYear();
  const isNegativeYear = yearN <= 0;
  const year = isNegativeYear ? pad(1 - yearN, 6) : pad(yearN, 6);
  const signedYear = (isNegativeYear ? "-" : "+") + year;

  const month = pad(date.getUTCMonth() + 1);
  const day = pad(date.getUTCDate());
  const hour = pad(date.getUTCHours());
  const minute = pad(date.getUTCMinutes());
  const second = pad(date.getUTCSeconds());
  const ms = pad(date.getUTCMilliseconds(), 3);

  return `${signedYear}-${month}-${day}T${hour}:${minute}:${second}.${ms}Z`;
}

const pad = (num: number, len = 2) => String(Math.abs(num)).padStart(len, "0");