const MONTH_FMT = new Intl.DateTimeFormat('en-US', { month: 'short', year: 'numeric', timeZone: 'UTC' });

function formatMonthYear(date: Date): string {
  return MONTH_FMT.format(date);
}

/**
 * Formats a PocketBase date string into a localized month+year range or "Present".
 * @throws {Error} if any date is invalid or if the range is logically impossible.
 */
export function formatMonthYearRange(start: string, end: string | null): string {
  const startDate = new Date(start);
  if (Number.isNaN(startDate.getTime())) {
    throw new Error('Invalid start date');
  }

  if (end === null) {
    return `${formatMonthYear(startDate)} — Present`;
  }

  const endDate = new Date(end);
  if (Number.isNaN(endDate.getTime())) {
    throw new Error('Invalid end date');
  }

  if (startDate > endDate) {
    throw new Error('Start date cannot be after end date');
  }

  const startLabel = formatMonthYear(startDate);
  const endLabel = formatMonthYear(endDate);

  if (startLabel === endLabel) {
    return startLabel;
  }

  return `${startLabel} — ${endLabel}`;
}