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

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.oneOfType([PropTypes.object, PropTypes.string]),
};

export default DatePreview;