方法一：正则表达式

1、年月日转换

var oldDate = "20111220";

var newDate = oldDate.replace(/^(\d{4})(\d{2})(\d{2})$/, "$1-$2-$3");

console.log(newDate );//2011-12-20

2、年月日时分秒转换

var oldDate = "20111220101010";

var newDate = oldDate.replace(/^(\d{4})(\d{2})(\d{2})(\d{2})(\d{2})(\d{2})$/, "$1-$2-$3 $4:$5:$6");

console.log(newDate );//2011-22-33 10:10:10

方法二：字符串处理

function formatStringyyyyMMddToyyyy_MM_dd (value){
    if(value.length == 8){
        return value.substring(0, 4) + "-" + value.substring(4, 6) + "-" + value.substring(6, 8);
    } else if(value.length == 6){
        return value.substring(0, 4) + "-" + value.substring(4, 6);
    } else {
    return value;
    }
}