java 获取前1个小时或者一个月

public static void getTwoHoursSith(Date mydate){
String lasttime="";
SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String str_date=sdf.format(mydate);
System.out.println(str_date);
mydate.setMonth(mydate.getMonth()-1);
// mydate.setHours(mydate.getHours()-2);
lasttime=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(mydate);
System.out.println(lasttime);
}