30. Menadżer subskrypcji

// Obliczenia kosztów
function calculateMonthlyCost(subscription) {
  const { price, billingCycle } = subscription;
  switch (billingCycle) {
    case 'monthly': return price;
    case 'quarterly': return price / 3;
    case 'yearly': return price / 12;
    default: return price;
  }
}

function getDaysUntilRenewal(renewalDate) {
  const diff = new Date(renewalDate) - new Date();
  return Math.ceil(diff / (1000 * 60 * 60 * 24));
}