日期对象|Date构造函数|

var date = new Date();  

console.log(date);   //Date {Wed Dec 10 2014 15:59:24 GMT+0800}

date.getDay()   // 3

date.setDate(1);
date.getDay();  // 1
console.log(date);   //Date {Mon Dec 01 2014 15:59:24 GMT+0800}

date.setMonth(0);
console.log(date);  //Date {Wed Jan 01 2014 15:59:24 GMT+0800}

火狐firebug控制台截图