SQL UDFs don't support SELECTs in 15, too.
Of course you could write a C/Java UDF returning a result set, just don't ask me about the details, that's not my topic :-)
But are you sure you actually need it, can you explain what you actually try to do?
I recommend my blog entry on this topic: http://developer.teradata.com/blog/georgecoleman/2014/01/ordered-analytical-functions-translating-sq... It seems to me impractical to allow functions to issue SQL in a parallel processing architecture. This blog entry explains how to implement that function-ality (so to speak) in a derived table.
Otherwise, in general, one can create a Table Operator (see the SQL External Routine Programming manual) to read from a table and produce another table as output, and you could adapt this Java code as a table operator. I am not sure how the users are using this converted date, but I am sure that this functionality could be done, as either a derived table or a table operator.