Javascrip中t获取指定日期的方法

Date.prototype.Format = function (fmt) {
 var o = {
 "M+": this.getMonth() + 1, //月份
 "d+": this.getDate(), //日
 "h+": this.getHours(), //小时
 "m+": this.getMinutes(), //分
 "s+": this.getSeconds(), //秒
 "q+": Math.floor((this.getMonth() + 3) / 3), //季度
 "S": this.getMilliseconds() //毫秒
 };
 if (/(y+)/.test(fmt)) fmt = fmt.replace(RegExp.$1, (this.getFullYear() + "").substr(4 - RegExp.$1.length));
 for (var k in o)
 if (new RegExp("(" + k + ")").test(fmt)) fmt = fmt.replace(RegExp.$1, (RegExp.$1.length == 1) ? (o[k]) : (("00" + o[k]).substr(("" + o[k]).length)));
 return fmt;
}

function getdate(days) {
 var now = new Date();
 var date = new Date(now.getTime() - days * 24 * 3600 * 1000);
 var dates = date.Format('yyyy-MM-dd');
 return dates;
}

function today_btn() {
 var today = getdate(0);
 var tomorrow = getdate(-1);
 document.getElementById("startdate").value = today;
 document.getElementById("enddate").value = tomorrow;
}

function yestoday_btn() {
 var today = getdate(0);
 var yestoday = getdate(1);
 document.getElementById("startdate").value = yestoday;
 document.getElementById("enddate").value = today;
}