js获取上个月时间

var myDate = new Date();    var year = myDate.getFullYear();    var month = myDate.getMonth();    if (month<10){        month = "0"+month;    }    var firstDay = month+"/"+"01/"+year;alert(firstDay);    myDate = new Date(year,month,0);    var lastDay = month+"/"+myDate.getDate()+"/"+year;   // var firstTime = "00:00:00";    //var lastTime = "23:59:59"; alert(lastDay); http://www.studyofnet.com/news/1180.html