const iframe = document.createElement('iframe');

document.body.appendChild(iframe);

const iWindow = iframe.contentWindow;

iWindow.document.write(`<script>var date = new Date()</script>`);

​

const checkedInstanceOfDate = iWindow.date instanceof Date;

const checkedPrototypeOfDate = Date.prototype.isPrototypeOf(iWindow.date);

const checkedSolutionFromSO = isValidDate(iWindow.date);

​

console.log(`x instanceof Date ⇒ ${checkedInstanceOfDate}`);

console.log(`Date.prototype.isPrototypeOf(x) ⇒ ${checkedPrototypeOfDate}`);

console.log(`Object.prototype.toString.call(x) === '[object Date]' ⇒ ${checkedSolutionFromSO}`);

​

function isValidDate(date) {

  return date && Object.prototype.toString.call(date) === "[object Date]" && !isNaN(date);

}