Try this:
String myDate= new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
.format(new Date(19000101 * 1000L));
Assuming it is the time since 1/1/1970
EDIT:-
If you want to convert from YYYYMMDD to YYYY-MM-DD format
Date dt = new SimpleDateFormat("yyyyMMdd", Locale.ENGLISH).parse(String.ValueOf(19000101));