fix(core): Stringify all Luxon DateTimes in cleanupParameterData (#8959)

packages/core/src/NodeExecuteFunctions.ts

@@ -2112,7 +2112,7 @@ export function cleanupParameterData(inputData: NodeParameterValueType): void {
 		(Object.keys(inputData) as Key[]).forEach((key) => {
 			const value = inputData[key];
 			if (typeof value === 'object') {
-				if (value instanceof DateTime) {
+				if (DateTime.isDateTime(value)) {
 					// Is a special luxon date so convert to string
 					inputData[key] = value.toString();
 				} else {

packages/core/test/NodeExecuteFunctions.test.ts

@@ -30,6 +30,7 @@ import { tmpdir } from 'os';
 import { join } from 'path';
 import Container from 'typedi';
 import type { Agent } from 'https';
+import toPlainObject from 'lodash/toPlainObject';
 
 const temporaryDir = mkdtempSync(join(tmpdir(), 'n8n'));
 
@@ -425,6 +426,16 @@ describe('NodeExecuteFunctions', () => {
 			expect(typeof input.y).toBe('string');
 		});
 
+		it('should stringify plain Luxon dates in-place', () => {
+			const input = {
+				x: 1,
+				y: toPlainObject(DateTime.now()),
+			};
+			expect(typeof input.y).toBe('object');
+			cleanupParameterData(input);
+			expect(typeof input.y).toBe('string');
+		});
+
 		it('should handle objects with nameless constructors', () => {
 			const input = { x: 1, y: { constructor: {} } as NodeParameterValue };
 			expect(typeof input.y).toBe('object');