Android : Fit the calendar format

I want mask yyyy/mm/dd

so.. I have used this code.

SimpleDateFormat format = new SimpleDateFormat("yyyy/MM/dd");
Calendar today = new GregorianCalendar();
String x = today.get(Calendar.YEAR)+"/"+today.get(Calendar.MONTH)+"/"+today.get(Calendar.DATE);
String a = format.format(x);
system.out.println(a);

I using this code But this was exception output.

08-14 15:24:36.327: E/AndroidRuntime(10133): java.lang.IllegalArgumentException

The exception here is the output.

String a = format.format(x);

the x output 2014/8/14. it mask yyyy/m/d

What do I need to fix?