Creating A Trigger To Compare Dates

I've been assigned a task at work that consists of creating a trigger on a table. This table is used to store temporary query results. I'm trying to make a trigger to clean the table so old results don't accumulate.

There is a column named DATE_UPDATED that stores an Oracle Date for when the row was inserted/updated. If the row is not updated in 12 hours, I want to delete it.

Here is what I have so far.

CREATE OR REPLACE TRIGGER clean_tableName
AFTER INSERT OR UPDATE
ON tableName

BEGIN

END clean_tableName;

As you can see, I don't have an actual function body yet. What would be the best way to accomplish this? Should I declare a variable to store the current Date+12 hours and then just compare that in a delete statement?

I'm thinking something like:

DECLARE
laterDate DATE := SYSDATE+12; // Not sure how to add 12 hours to a date.
BEGIN
DELETE FROM tableName WHERE DATE_UPDATED > laterDate;
END clean_tableName;

But I'm not sure how to add 12 hours to the current date. Is there a way I can do this without defining a variable?