This milk goes through much less processing than evaporated milk. It starts with pasteurized milk combined with a sugar solution. The water is then extracted until the mixture is less than half its original weight. It is not heated because the high sugar content prevents spoilage. It's very high in calories: 8 oz has 980 calories.
Although it is often hard to find, the label has a stamped date code which indicates the date by which it should be consumed. Sweetened condensed milk may thicken and darken as it ages, but it is still edible.
Check the date on sweetened, condensed milk for maximum storage.