If you're concerned about food safety, use your nose. If you bought raw chicken and you know it's spent 2 days in the refrigerator after the sell-by date, smell it. If it's bad, you'll know right away. It's the same for milk. It may be good for several days after the sell-by date, but if there's a sour smell, you know it's bad. Otherwise, make sure to keep items in their original packaging to keep track of the dates, or make sure you have a good labeling system so you're cooking chicken or other foods at peak freshness.