JAVA:int month = n.get(Calendar.MONTH)+1;为何要加1?

JAVA:int month = n.get(Calendar.MONTH)+1;为何要加1?
import java.util.*;
public class DateClass
{
public static void main(String args[ ])
{
Date m = new Date( );
System.out.println("现在是："+m.toString( ));
Calendar n = Calendar.getInstance( );
int year = n.get(Calendar.YEAR);
int month = n.get(Calendar.MONTH)+1;
int day = n.get(Calendar.DATE);
int week = n.get(Calendar.DAY_OF_WEEK)-1;
Date t = n.getTime ( );
String week1 = new String("日一二三四五六");
String week2=week1.substring(week,week+1);
String x = new String( );
x = "现在是："+year+"年"+month+"月"+day+"日,星期"+week2;
System.out.println("现在是："+t);
System.out.println(x);
}
}
为什么定义month和week时要加1和减1?而定义day时不用啊?substring(week,week+1)具体啥意思啊?
偶是个新手,问题有些笨,还希望不吝赐教啊!