Try looking up the SESSION system handle in the documentation. There you will discover that the date-format attribute is both readable and writable.
BTW -- if you are running Windows there is a very handy "Help" program installed with Progress. I like to add it to the "quick launch" toolbar. You might be amazed at how handy it is. I keep it open to the "Index" tab most of the time. It makes it really easy to look stuff up.