//dateObj是一個日期對象，days表示給這個日期加多少天，比如說4，5（天）
function dateAdd(dateObj,days){
var tempDate = dateObj.valueOf();
tempDate = tempDate + days * 24 * 60 * 60 * 1000;
tempDate = new Date(tempDate);
return tempDate;
}

//然后使用,創建一個日期對象
var dateValue = document.getElementById("XXYY").value.split("-");
var custArvDateTwoValue = new Date(dateValue[0],dateValue[1]-1,dateValue[2]);
//調用dateAdd，加兩天
custArvDateTwoValue = dateAdd(custArvDateTwoValue,2);
var year = custArvDateTwoValue.getFullYear();
var month = custArvDateTwoValue.getMonth() + 1;
var days = custArvDateTwoValue.getDate();
month = month <= 9 ? "0"+month : month;
days = days <= 9 ? "0"+days : days;
document.getElementById("XX").value = year + "-" + month + "-" +days;