I have to do a "select" that returns the difference between a field of type date from the database and the current date in hours. How can I do this in oracle?

I try this:

```
   select 24 * (to_date(sysdate, 'YYYY-MM-DD hh24:mi') 
         - to_date('2012-02-28 15:20', 'YYYY-MM-DD hh24:mi')) diff_hours 
   from dual;

```

But that did not work.