exp:根据日期计算是一年中的第几天？
<body>
<input type="text" id="a1"/>
<input type="button" id="btn9" value="天数"/>
</body>
<script>
btn9.onclick = function (){
var a= a1.value;
var year =parseInt(a/10000);
var month =parseInt((a/100)%100);
var day =parseInt(a%100);
var run;
console.log(month,day)
if (year%400==0||(year%4==0&&year%100!=0)) {
run=29;
}else{
run=28;
}
switch(month){
case 12:
day+=30;
case 11:
day+=31;
case 10:
day+=30;
case 9:
day+=30;
case 8:
day+=31;
case 7:
day+=30;
case 6:
day+=31;
case 5:
day+=30;
case 4:
day+=31;
case 3:
day+=run;
case 2:
day+=31;
case 1:
alert('第'+day+'天');
}
}
</script>