By default the month is a text value in the select list (Jan, Feb, March etc.), in case you want to show a number instead of the text use this. Hope this helps someone!
Code:
final LinkedHashMap<String, String> monthValueMap = new LinkedHashMap<String, String>();
monthValueMap.put("0", "1");
monthValueMap.put("1", "2");
monthValueMap.put("2", "3");
monthValueMap.put("3", "4");
monthValueMap.put("4", "5");
monthValueMap.put("5", "6");
monthValueMap.put("6", "7");
monthValueMap.put("7", "8");
monthValueMap.put("8", "9");
monthValueMap.put("9", "10");
monthValueMap.put("10", "11");
monthValueMap.put("11", "12");
final SelectItem monthSelectItem = new SelectItem("Month");
monthSelectItem.setValueMap(monthValueMap);
final DateItem nextShipment = new DateItem();
nextShipment.setName("nextShipment");
nextShipment.setMonthSelectorProperties(monthSelectItem);
...