You can make a query on ceatedOn by using CREATED_ON_KEY as "_$createdAt" .For this you need to follow the code snippet given below :-
Query q1 = QueryBuilder.build("_$createdAt", "2014-05-15", Operator.GREATER_THAN);
And if you are using the latest version of our java sdk then you can use the method setCreatedOn for searching document based upon createdOn. For this you need to follow the code snippet given below :-
Query q1 = QueryBuilder.setCreatedOn("2014-10-10", Operator.GREATER_THAN_EQUALTO);
Please let us know if this helps.