js计算前三个月的时间精确到时分秒

Date.prototype.format = function(format) {
var o = {
"M+" : this.getMonth() + 1, // month
"d+" : this.getDate(), // day
"h+" : this.getHours(), // hour
"m+" : this.getMinutes(), // minute
"s+" : this.getSeconds(), // second
"q+" : Math.floor((this.getMonth() + 3) / 3), // quarter
"S" : this.getMilliseconds()
// millisecond
}
if (/(y+)/.test(format))
format = format.replace(RegExp.$1, (this.getFullYear() + "").substr(4 - RegExp.$1.length));
for ( var k in o)
if (new RegExp("(" + k + ")").test(format))
format = format.replace(RegExp.$1, RegExp.$1.length == 1 ? o[k] : ("00" + o[k]).substr(("" + o[k]).length));
return format;
}

var begin=new Date();
begin.setMonth((new Date().getMonth()-3));
//var begintime= begin.format("yyyy-MM-dd 00:00:00");零点格式
var begintime= begin.format("yyyy-MM-dd hh:mm:ss");
$('#beginDate').val(begintime);