Java SimpleDateFormat Bug?

Java SimpleDateFormat Bug?

Dose anyone know why I get below output? I expected 2012-12 instead of
2013-12
Calendar cal=Calendar.getInstance();
cal.set(Calendar.YEAR,2012);
cal.set(Calendar.MONTH,12-1);
cal.set(Calendar.DATE,30);
Date date=cal.getTime();
System.out.println(date);
String YYYY_MM=format.format(date);
System.out.println(YYYY_MM);
OUTPUT
Sun Dec 30 18:30:52 KST 2012
2013-12
EXPECTED
Sun Dec 30 18:30:52 KST 2012
2012-12