fix: properly handle Date objects in form.fields.set

.changeset/fix-date-form-fields-set.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: properly handle Date objects in form.fields.set

packages/kit/src/runtime/form-utils.js

@@ -617,6 +617,10 @@ function get_type_prefix(field_type, is_array, input_value) {
  */
 function deep_clone(value) {
 	if (value !== null && typeof value === 'object') {
+		if (value instanceof Date) {
+			return new Date(value.getTime());
+		}
+
 		if (value instanceof File) {
 			return value;
 		}

packages/kit/src/runtime/form-utils.spec.js

@@ -2,6 +2,7 @@ import { describe, expect, test } from 'vitest';
 import {
 	BINARY_FORM_CONTENT_TYPE,
 	convert_formdata,
+	create_field_proxy,
 	deep_set,
 	deserialize_binary_form,
 	serialize_binary_form,
@@ -745,3 +746,27 @@ describe('deep_set', () => {
 		expect(target).toEqual({ nested: { name: 'hello' } });
 	});
 });
+
+describe('create_field_proxy', () => {
+	// Regression test for https://github.com/sveltejs/kit/issues/16165
+	// Before the fix, Date values fell through to the generic object branch
+	// of deep_clone, which iterated Object.keys (empty on Date), producing
+	// an empty plain object {} instead of a Date.
+	test('value() returns a Date clone for Date fields', () => {
+		const original = new Date('2025-06-25T00:00:00Z');
+		const input = { created_at: original };
+
+		const proxy = create_field_proxy(
+			{},
+			() => input,
+			() => {},
+			() => ({})
+		);
+
+		const cloned = proxy.created_at.value();
+
+		expect(cloned).toBeInstanceOf(Date);
+		expect(cloned.getTime()).toBe(original.getTime());
+		expect(cloned).not.toBe(original);
+	});
+});