var today = new Date();
var weekDays = new Array("日","月","火","水","木","金","土");

function dateString(today) {
var Day = weekDays[today.getDay()]
var Month = today.getMonth()+1;
return today.getFullYear() + "年" + Month + "月" + today.getDate() + "日(" + Day + ")";
}

var now = new Date();
var nYear = now.getFullYear();
var nMonth = now.getMonth();
var nDate = now.getDate();

var yesterday = new Date(nYear, nMonth, nDate - 1);
var yYear = yesterday.getFullYear();
var yMonth = yesterday.getMonth();
var yDate = yesterday.getDate();