Help with Date Program

Question

Okay, being a complete n00b in java, but moderatley experienced in c, i have found arrays in java to be slightly different in java. The following program returns errors on lines 61, 26, and 14. whats wrong with it?
import javax.swing.JOptionPane;
class Practical3{
     public static void main(String [] args){
          int day, month, year;
          boolean isDateValid;
          String Sday, Smonth, Syear;
          Sday=JOptionPane.showInputDialog("Please enter the day");
          day=Integer.parseInt(Sday);
          Smonth=JOptionPane.showInputDialog("Please enter the month");
          month=Integer.parseInt(Smonth);
          Syear=JOptionPane.showInputDialog("Please enter the year");
          year=Integer.parseInt(Syear);
          isDateValid=validDate(day,month);
          if(isDateValid==true)
               System.out.println("The date is:"+day+"/"+month+"/"+year);
          else
               System.out.println("The date is not valid");
          return;
     public static boolean validDate(int validateday, int validatemonth){
          boolean checkDate;
          int x;
          int y;
          x=validMonth(validatemonth);
          y=validDay(validateday, validatemonth);
          if(x==1 && y==1)
               checkDate=true;
          else
               checkDate=false;
          return checkDate;
     public static int validMonth(int checkMonth){
          int validation;
          if(checkMonth>=1 && checkMonth<=12)
               validation=1;
          else
               validation=0;
          return validation;
     public static int validDay(int dayCheck, int appmonthCheck){
          int[] runner;
          int rvalue;
          int curmonth;
          curmonth=(appmonthCheck-1);
          runner= new int[12];
          runner[0]=31;
          runner[1]=28;
          runner[2]=31;
          runner[3]=30;
          runner[4]=31;
          runner[5]=30;
          runner[6]=31;
          runner[7]=31;
          runner[8]=30;
          runner[9]=31;
          runner[10]=30;
          runner[11]=31;
          if(dayCheck<=runner[curmonth])
               rvalue=1;
          else
               rvalue=0;
          return rvalue;
}

Answer 1

mm, okay, the program compiles fine, but when i execute it and enter values, i SOMETIMES get the following error:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 54
at Practical3.validDay(Practical3.java:61)
at Practical3.validDate(Practical3.java:26)
at Practical3.main(Practical3.java:14)
Press any key to continue...
line 14 says "isDateValid=validDate(day,month);"
line 26 says "y=validDay(validateday, validatemonth);"
line 61 says "if(dayCheck<=runner[curmonth])"
now note that i said it only happens SOMETIMES, as there may be another time when i execute the code and i can enter the values and they are displayed correctly, ie the program works. Has this ever happened someone before? Im using the latest jdk and jcreator btw...