export function todayIso(): string {
  return new Date().toISOString().slice(0, 10);
}

export function addDaysIso(iso: string, days: number): string {
  const d = new Date(`${iso}T12:00:00`);
  d.setDate(d.getDate() + days);
  return d.toISOString().slice(0, 10);
}

export function startOfWeekIso(iso: string): string {
  const d = new Date(`${iso}T12:00:00`);
  const day = d.getDay();
  const diff = day === 0 ? -6 : 1 - day;
  d.setDate(d.getDate() + diff);
  return d.toISOString().slice(0, 10);
}

export function endOfWeekIso(iso: string): string {
  return addDaysIso(startOfWeekIso(iso), 6);
}

export function formatDisplayDate(iso: string, locale: string): string {
  return new Date(`${iso}T12:00:00`).toLocaleDateString(locale, {
    weekday: "short",
    month: "short",
    day: "numeric",
  });
}