SQL & PL/SQL

I know it is basic stuff, but I am not sure how to handle it.
I have a function A which returns date. The problem I am having is I want to use the output of function A as a input of Function B. for example;

Procedure abc
xx number;
yy number;
res date;

begin
case when xx=1 then function A(xx)
when xx=2 then function A(yy)
end as res,

function B (res);
end procedure;

What is the best way to save the output of one function into a variable and then use it as a input parameter to other function?

Thanks in advance.