用javascript实现一个简单的日历

<div class="box">
    <div class="date">
        <div class="title">
            <span class="last" id="showLastMonth"><</span>
            <div class="current">
                <span id="showYear"></span>
                <span>/</span>
                <span id="showMonth"></span>
            </div>
            <span class="next" id="showNextMonth">></span>
        </div>
        <div class="xq">
            <ul>
                <li>日</li>
                <li>一</li>
                <li>二</li>
                <li>三</li>
                <li>四</li>
                <li>五</li>
                <li>六</li>
            </ul>
        </div>
        <div class="day">
            <ul id="showDay">
            </ul>
        </div>
    </div>
</div>

ul{list-style: none;margin: 0;padding: 0;overflow: hidden;}
ul li{float: left;width: 30px;height: 30px;line-height: 30px;margin: 5px;text-align: center;border-radius: 50%;}
ul .today{background: #FD6639;color: #FFF;}
.day li{font-size: 15px;}
.day li:hover{color: #00F;cursor: pointer;}
.box{width: 280px;border: 1px solid #DDD;}
.xq{background: #E7F1FD;}
.title{background: #7AB2ED;height: 40px;line-height: 40px;}
.title .last{float: left;width: 40px;text-align: center;cursor: pointer;}
.title .next{float: right;width: 40px;text-align: center;cursor: pointer;}
.title .current{text-align: center;width: 200px;float: left;color: #FFF;}
.title .last:hover{color: #FD6639;background: rgba(0,0,0,0.2);}
.title .next:hover{color: #FD6639;background: rgba(0,0,0,0.2);}

<script type="text/javascript">
    var date = new Date();
    var year = date.getFullYear();//年
    var month = date.getMonth();//月(0-11)
    var currentY = year;
    var currentM = month;
    var currentD = date.getDate();
	
    showDate(date);

    var lastBtn = document.getElementById("showLastMonth");
    var nextBtn = document.getElementById("showNextMonth");
    lastBtn.onclick = function(){
        if(month==0){
            year-=1;
            month=11;
        }else{
            month--;
        }
        showDate(new Date(year,month,1));
    }
    nextBtn.onclick = function(){
        if(month==11){
            year+=1;
            month=0;
        }else{
            month++;
        }
        showDate(new Date(year,month,1));
    }

    function showDate(obj){
        var year = obj.getFullYear();//年
        var month = obj.getMonth();//月
        //var date = obj.getDate();//日
        var sDay = new Date(year,month,1).getDay();//本月第一天星期几
        var days = new Date(year,month+1,0).getDate();//本月一共多少天
        document.getElementById("showYear").innerHTML = year;
        document.getElementById("showMonth").innerHTML = month+1;
        var dayObj = document.getElementById("showDay");
		
        var htmlStr = "";
        for (var i = 1; i <= sDay; i++) {
            htmlStr += "<li></li>";
        }
		
        for (var i = 1; i <= days; i++) {
            if(currentY == year && currentM == month && i == currentD){
                htmlStr += "<li class='today'>"+i+"</li>";
            }else{
                htmlStr += "<li>"+i+"</li>";
            }
        }
        dayObj.innerHTML = htmlStr;
    }
</script>