输入一个日期，判定该天是该年的第几天

<script type="text/javascript">

function days(year,month,day) {
var days = 0;
for(var i = 1; i < month; i++ ){
switch(i){
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:{
days += 31;
break;
}
case 4:
case 6:
case 9:
case 11:{
days += 30;
break;
}
case 2:{
if(isLeapYear(year)){
days += 29;
}
else {
days += 28;
}
break;
}

default:{
alert("月份有误");
}

}
}
days += day;
return days;
}
//判断年份是 闰年还是平年
function isLeapYear(year){
if( (year % 4 == 0 && year % 100 != 0)
|| year % 400 == 0){
return true;
}
else {
return false;
}
}
alert(days(2012,2,1));

</script>