fix(keystatic): reject non-midnight UTC Date values in fields.date() parse (#1615)

.changeset/date-field-preserve-non-midnight-instant.md

@@ -0,0 +1,5 @@
+---
+'@keystatic/core': patch
+---
+
+Avoid silently truncating non-midnight UTC `Date` values in `fields.date().parse()` (#1615).

packages/keystatic/src/form/fields/date/date.test.ts

@@ -0,0 +1,16 @@
+import { expect, test } from '@jest/globals';
+import { date } from './index';
+
+test('parses UTC midnight Date instances into YYYY-MM-DD', () => {
+  const field = date({ label: 'Publish Date', validation: { isRequired: true } });
+  const parsed = field.parse(new Date('2026-09-01T00:00:00.000Z'));
+  expect(parsed).toBe('2026-09-01');
+  expect(field.validate(parsed)).toBe('2026-09-01');
+});
+
+test('does not silently truncate Date instances carrying a non-zero UTC time (#1615)', () => {
+  const field = date({ label: 'Publish Date', validation: { isRequired: true } });
+  const parsed = field.parse(new Date('2026-09-01T13:00:00.000Z'));
+  expect(parsed).toBe('2026-09-01T13:00:00.000Z');
+  expect(() => field.validate(parsed)).toThrow('Publish Date is not a valid date');
+});

packages/keystatic/src/form/fields/date/index.tsx

@@ -52,6 +52,14 @@ export function date<IsRequired extends boolean | undefined>({
         return null;
       }
       if (value instanceof Date) {
+        if (
+          value.getUTCHours() !== 0 ||
+          value.getUTCMinutes() !== 0 ||
+          value.getUTCSeconds() !== 0 ||
+          value.getUTCMilliseconds() !== 0
+        ) {
+          return value.toISOString();
+        }
         const year = value.getUTCFullYear();
         const month = String(value.getUTCMonth() + 1).padStart(2, '0');
         const day = String(value.getUTCDate()).padStart(2, '0');