How to make a TIMESTAMP_ADD call using RelBuilder?

[Anton Haidai]

Hello. In SQL, I can execute a query like "SELECT TIMESTAMPADD(month,
1, "date" ) FROM ..." and it works. But my attempts to do the same
thing using RelBuilder are not successful, for example, this code does
not work:

RexNode shiftedDateField = relBuilder.call(
    SqlStdOperatorTable.TIMESTAMP_ADD,
    relBuilder.literal("month"),
    relBuilder.literal(1),
    dateFieldRef
);

Could you please tell me, how to make a TIMESTAMP_ADD call using RelBuilder?
-- 
Best regards,
Anton.