Sell by date good after cheese is open?

I opened up a pack of sliced swiss cheese and put it back in the fridge about 3 weeks ago. I looked at it today and it looks ok. The sell by/use by date is months away. Does that date apply after you open the package?|||As long as there is no green color on it... it should be fine... even after the sell by date!|||Once you open it you need to keep it sealed well to prevent drying out and mold. But you say Swiss cheese? LOL It will stink to high heaven when it goes bad. It should be just fine.