Line 85 of DropdownDatePicker.java:
json.put("date", getTranslatedFieldSupport().format(this, getValue()));
however, this generates these errors in the browser from DoJo:
DEBUG: invalid date string: 10/02/06
DEBUG: DEPRECATED: dojo.widget.DropdownDatePicker date attributes must be passed in Rfc3339 format – will be removed in version: 0.5
This is because I defined a custom Translator for the field. It is picking up it's format string for the parse/validation phase, but the date value is the initial displayed value used when generating the text box, and needs to be in "yyyy-MM-dd" format.
When changing this, don't use a static DateFormat, of course, as they aren't thread safe. Been bitten by that before - took a serious debug effort to track that one down. You can use an Object pool or ThreadLocal, however.