var Calendar = Class.create();

Calendar.prototype = {

    initialize : function(e, obj) {

        this.event = (e) ? e : window.event;
        this.obj = obj;

        this.today = this.today = new Date();
        this.year = this.today.getFullYear();
        this.month = this.today.getMonth();
        this.date = 1;


        this.dayIndex = new Array('일','월','화','수','목','금','토');
        this.lastDate = new Array(31,  28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

        this.calWin = window.open('', 'calendar', 'width=200,height=180,top='+(this.event.clientY+100)+',left='+this.event.clientX);
        this.makeCal();
    },

    makeCal : function(type, addSub) {
        this.newDate = new Date(this.year, this.month, this.date);
        if(type && addSub) {
            if(type == 'year') {
                if(addSub == 'add') this.newDate.setFullYear(this.newDate.getFullYear()+1); else this.newDate.setFullYear(this.newDate.getFullYear()-1);
            } else if(type == 'month') {
                if(addSub == 'add') this.newDate.setMonth(this.newDate.getMonth()+1); else this.newDate.setMonth(this.newDate.getMonth()-1);
            }
        }

        this.year = this.newDate.getFullYear();
        this.month = this.newDate.getMonth();
        this.day = this.newDate.getDay();

        if(((this.year%4 == 0) && (this.year%100 != 0)) || (this.year%400 ==0)) {
            this.lastDate[1] = 29;
        } else {
            this.lastDate[1] = 28;
        }

        _cal = this;

        this.calsrc = "<table border='0' cellpadding='0' cellspacing='1' borderColor='#eeeeee' width='100%'>\n";
        this.calsrc += "<tr>\n";
        this.calsrc += "<td><span onClick=opener._cal.makeCal('year','sub')><<</span> <span onClick=opener._cal.makeCal('month','sub')><</span></td>\n";
        this.calsrc += "<td>"+this.newDate.getFullYear()+"년 "+(this.month+1)+"월</td>\n";
        this.calsrc += "<td><span onClick=opener._cal.makeCal('month','add')>></span> <span onClick=opener._cal.makeCal('year','add')>>></span></td>\n";
        this.calsrc += "</tr>\n";
        this.calsrc += "</table>\n";
        this.calsrc += "<table border='0' cellpadding='0' cellspacing='1' borderColor='#eeeeee' width='100%'>\n";
        this.calsrc += "<tr>\n";
        for(i=0; i<this.dayIndex.length; i++) {
            this.calsrc += "<td>"+this.dayIndex[i]+"</td>\n";
        }
        this.calsrc += "</tr>\n";
        for(i=0; i<this.day+this.lastDate[this.newDate.getMonth()];i++) {
            if(!(i%7)) {
                this.calsrc += "</tr>\n";
                this.calsrc += "<tr>\n";
            }

            if(i >= this.day) {
                this.calsrc += "<td><span onClick=opener._cal.setDate('"+((i-this.day)+1)+"')>"+((i-this.day)+1)+"</td>\n";
            } else {
                this.calsrc += "<td></td>\n";
            }
        }
        this.calsrc += "</tr>\n";
        this.calsrc += "</table>\n";

        this.calWin.document.open();
        this.calWin.document.clear();
        this.calWin.document.write(this.calsrc);
        this.calWin.document.close();
    },

    fillZero : function(len, value) {
        var value = value.toString();
        var addLen = len - value.length;
        for(i=0; i<addLen; i++) {
            value = "0"+value;
        }
        return value;
    },

    setDate : function(day) {
        $(this.obj).value = this.year+'-'+this.fillZero(2,this.month+1)+'-'+this.fillZero(2,day);
        this.calWin.close();
    }
}



<input type="text" name="date" id="date'><input type="button" onClick="new Calendar(event, 'date')" value="달력" />

이렇게 사용하면 됩니다.
급조하느라 부족한부분이 많습니다.