variables like $MyDatetimeFieldFilter don't exist in Java. In Java you need to set the date filter as a number value. Beware of the fact that for dates you need a range in most cases.
Long L = MyDatetimeFieldFilter.getTime();
List<IMendixObject> results = Core.createXPathQuery("//MYModule.Myentity[MyDatetimeField=”+ L + “]")