js:

//獲取當前日期
var myDate = new Date();
var nowY = myDate.getFullYear();
var nowM = myDate.getMonth()+1;
var nowD = myDate.getDate();
var enddate = nowY+"-"+(nowM<10 ? "0" + nowM : nowM)+"-"+(nowD<10 ? "0"+ nowD : nowD);//當前日期

//獲取三十天前日期
var lw = new Date(myDate - 1000 * 60 * 60 * 24 * 30);//最後一個數字30可改，30天的意思
var
 
 lastY = lw.getFullYear();
var lastM = lw.getMonth()+1;
var lastD = lw.getDate();
var startdate=lastY+"-"+(lastM<10 ? "0" + lastM : lastM)+"-"+(lastD<10 ? "0"+ lastD : lastD);//三十天之前日期

===========================================================================================================

java:

//獲取當前日期
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date today = new Date();
String endDate = sdf.format(today);//當前日期

//獲取三十天前日期
Calendar theCa = Calendar.getInstance();
theCa.setTime(today);
theCa.add(theCa.DATE, -30);//最後一個數字30可改，30天的意思
Date start = theCa.getTime();
String startDate = sdf.
 
format(start);//三十天之前日期

2018年5月23日更新：

/**
 * 在原有的日期上往後推，計算得出新的日期.
 * 
 * @param billingDate 日期
 * @param days 天數
 * @return 計算後的日期
 */
public static java.sql.Date dateAfter(Date billingDate, int days) {
    Calendar cal = Calendar.getInstance();
    cal.setTime(billingDate);
    cal.add(Calendar.DATE, days);
    java.sql.Date date = new java.sql.Date(cal.getTime().getTime());
    return date;
}