js 获取昨日时间

Date.prototype.format = function(format) {
  var date = {
      "M+": this.getMonth() + 1,
      "d+": this.getDate(),
      "h+": this.getHours(),
      "m+": this.getMinutes(),
      "s+": this.getSeconds(),
      "q+": Math.floor((this.getMonth() + 3) / 3),
      "S+": this.getMilliseconds()
 };
 if (/(y+)/i.test(format)) {
        format = format.replace(RegExp.$1, (this.getFullYear() + '').substr(4 -         RegExp.$1.length));
 }
 for (var k in date) {
     if (new RegExp("(" + k + ")").test(format)) {
        format = format.replace(RegExp.$1, RegExp.$1.length == 1? date[k] : ("00" + date[k]).substr(("" + date[k]).length));
     }
  }
  return format;
};
 document.write((new Date()).format('yyyy-MM-dd h:m:s'));

var timestramp = (new Date()).getTime();
var yestoday = new Date(timestramp-60*60*24*1000);
var time = yestoday.getFullyear()+'-'+Number(yestoday.getMonth())+1+'-'+yestoday.getDate();