javascript - 关于setTime()的用法实际与w3school有差异？？？

Question

D=new Date();
D1=D.setTime(77771564221);
alert(D1)//77771564221??
应该输出Mon Jun 19 1972 11:12:44 GMT+0800 (中国标准时间)？？？
按照w3school的例子写的

Answer 1

<script type="text/javascript">

var d = new Date()
d.setTime(77771564221)
document.write(d)

</script>