String.prototype.parseDate = function(){
    var ar = (this + ",0,0,0").match(/\d+/g);
    return ar[5]?(new Date(ar[0],ar[1]-1,ar[2],ar[3],ar[4],ar[5])):(new Date());
}
Date.prototype.format = function(format) {

    /*
    * eg:format="yyyy-MM-dd hh:mm:ss";
    */
    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 mydate = document.getElementById("Begintime").value.parseDate();
mydate.setMinutes(mydate.getMinutes() + 40);
alert(mydate.format("yyyy-MM-dd hh:mm:ss"))