Usage:

const targetDate = new Date('2025-12-31').toISOString();
const { days, hours, minutes, seconds, isExpired } = useCountdown(targetDate);

return (
  <div>
    {isExpired ? (
      <span>Timer expired!</span>
    ) : (
      <span>
        {days}d {hours}h {minutes}m {seconds}s
      </span>
    )}
  </div>
);