migrate date and datetime widgets

packages/netlify-cms-widget-date/src/DatePreview.js

@@ -0,0 +1,13 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { WidgetPreviewContainer } from 'netlify-cms-ui-default';
+
+const DatePreview = ({ value }) => (
+  <WidgetPreviewContainer>{value ? value.toString() : null}</WidgetPreviewContainer>
+);
+
+DatePreview.propTypes = {
+  value: PropTypes.object,
+};
+
+export default DatePreview;