Diet Coke lasts for six to nine months past its printed date in the pantry if unopened. The soft drink lasts for the same amount of time if refrigerated and unopened.
If a bottle of Diet Coke is left open, it lasts in the refrigerator or the pantry for one day before its carbon dioxide is completely released and it goes flat; however, it is still safe to drink. Diet Coke contains artificial sweetener, and, after some time, the sweetener breaks down. If the soda differs in taste, this is a sign that it is spoiled and should be discarded immediately.