This is a Perl oriented site, not PL/SQL...the correct place for PL/SQL questions is http://metalink.oracle.com or perhaps http://docs.oracle.com. However, I can answer the question here - the value returned is a number:

set serveroutput on
declare
d1 date;
d2 date;
n1 number;
begin
d1 := sysdate;
d2 := sysdate - 1;
n1 := d1 - d2;
dbms_output.put_line(n1);
end;
/
[download]

results in "1", and just to prove it's not a coerced value, if you change the type of n1 to date, it doesn't compile.

rdfield