Automatically calculate the date oracle APEX

how can the date be calculated depending on the input of a user? That is, depending on what the user enters, 3 months will be added to the current date or 5, etc.

I tried to add a computation to the date pickers:

• SUBMISSION_DATE displays only the current date (already implemented)
• DATE_OF_ISSUE should be SUBMISSION_DATE +3months or +5months, depending on users input in another field.

I added as well a computation to this date with the following pl/sql function body, but it is not working.

DECLARE
sub_date DATE default null;
BEGIN
if :P75_THESISTYPE =1
then 
sub_date := :P75_SUBMISSION_DATE +6;
else
sub_date := :P75_SUBMISSION_DATE +3;
END IF;
return sub_date;
end;

@TonyAndrews has the correct process, but as he followed your initial statement, it will not give the desired result. Adding an integer to a data add that number of days not months. What you need is the ADD_Months function; So:

DECLARE
  sub_date DATE default null;
BEGIN
  if :P75_THESISTYPE =1
  then 
    sub_date := add_months(to_date(:P75_SUBMISSION_DATE),6);
  else
    sub_date := add_months(to_date(:P75_SUBMISSION_DATE),3);
  end if
  return sub_date;
end;
/

Of course the above assumes your variable's format matches your nls_data_format. Generally a dangerous assumption at best,