Handle Date Math That Expressions Cannot

n8n's built-in expression language supports Luxon for dates, but complex operations like business day calculations, timezone-aware scheduling windows, or fiscal quarter logic are easier and more maintainable in a Code node.

Real-world example: A contract renewal workflow needs to calculate the next business day that is at least 30 days out, skipping US federal holidays and weekends.

// Mode: Run Once for Each Item

// US federal holidays for the current year (update annually or fetch dynamically)
const holidays2025 = new Set([
  '2025-01-01', '2025-01-20', '2025-02-17', '2025-05-26',
  '2025-06-19', '2025-07-04', '2025-09-01', '2025-10-13',
  '2025-11-11', '2025-11-27', '2025-12-25',
]);

function isBusinessDay(date) {
  const day = date.getDay();
  if (day === 0 || day === 6) return false; // Weekend
  const iso = date.toISOString().slice(0, 10);
  return !holidays2025.has(iso);
}

function addBusinessDays(start, days) {
  const result = new Date(start);
  let added = 0;
  while (added < days) {
    result.setDate(result.getDate() + 1);
    if (isBusinessDay(result)) added++;
  }
  return result;
}

const contractEnd = new Date($input.item.json.contractEndDate);
const minDate = new Date(contractEnd);
minDate.setDate(minDate.getDate() + 30);

// Find next business day on or after the 30-day mark
let renewalDate = new Date(minDate);
while (!isBusinessDay(renewalDate)) {
  renewalDate.setDate(renewalDate.getDate() + 1);
}

// Also calculate 5-business-day reminder window
const reminderDate = addBusinessDays(renewalDate, -5);

return [{
  json: {
    ...$input.item.json,
    renewalDate: renewalDate.toISOString().slice(0, 10),
    reminderDate: reminderDate.toISOString().slice(0, 10),
    daysUntilRenewal: Math.ceil(
      (renewalDate - new Date()) / (1000 * 60 * 60 * 24)
    ),
  }
}];
```text
This logic would require an unreasonable number of IF and DateTime nodes to replicate on the canvas.

**Related:** [Use Edit Fields in "Map Each" Mode for Simple Renames](../data-transformation/01-use-edit-fields-in-map-each-mode-for-simple-renames.md) | [Configure Payload Size and Binary Data Mode for Large Files](../performance-and-large-files/01-configure-payload-size-and-binary-data-mode-for-large-files.md)