Unfortunately the carton dates only indicate when the items were packed into those cartons, they may well have been in storage before being packed into the cardboard cartons. If you get the names of the producers off the cartons then you can usually find out a little more, if the carton is generic and does not carry the name of the producer then chances are it's been repacked somewhere along the line. Cardboard cartons don't hold up well in high humidity storage, so anything that has been stored has to be repacked when it comes out of storage....unless it was originally packed in breathable plastic....look at the little punnets of berries as an example.

My suggestions:-
1. Don't buy anything with artificial sweeteners in it - this is pretty much anything with the word "Diet" written on it. Artificial sweeteners have been repeatedly shown to stimulate appetite - if you eat something sweet that contains few calories, then your body will make you go looking for the calories elsewhere. Many reduced fat items are artificially sweetened. Use natural sugars - honey, fruit, etc in preference to processed sugar.

2. Eat for flavour as much as nutrition - treat your meals as a celebration of produce, not merely an exercise in sustenance. Commercial food production is about producing maximum product for minimum cost, the biggest cost item is naturally grown produce - very few traditional diets work this way - the bulk of your diet should consist of fresh produce, complemented with a small quantity of prepared stocks and sauces.

3. Make your own stocks and sauces - it's the best way of getting maximum nutritional value out of the produce that have bought, the offcuts and trimmings from your more elaborate meals will furnish you with a range of sauces, stock and even soups. Don't buy pre-trimmed produce - if you buy root vegetables, then get them with the leaves still attached, buy whole chickens, whole fish, etc. Just about every part of the produce has a place in most traditional diets, the roughage, fibre and nutritional benefits of these sub-prime foods goes a long way to balance your diet.
Prepared sauces and dressings are usually really bad news, they are salt, sugar and fat rich and full of chemicals to stablise them for long term storage. If you have oils, spices and vinegars in your store cupboard, then you can reconstruct most of the supermarkets' product ranges with surprisingly little effort.

4. Source your produce carefully - know where it has come from, how it has been handled and treated. Avoid varities of fruit and veg that have been geneticially modified for faster growth - slow, growing varieties contain more flavour and more nutritional benefit.

5. Source your meat with even more care - eat more mature animals on a regular basis, mutton or hoggett in preference to lamb, beef > 2 years old (most Aussie beef is <1 yr). Avoid animals that have been specially grown for fast growth, try to source meat that grows slower for more flavour.

6. Eat less fat, but don't compromise the rest of your diet to do it - if you increase the intake of sugar and processed grains in order to reduce fat then you will do more harm than good. Fats carry flavour and much nutritional value, eradicating them is not part of a balanced diet. Learn the lessons from traditional diets that use cooking methods and seasonings to get maximum benefit and minimum detriment from fat.

7. Season your food well - the use of a broad range of seasonings (fresh and dried) will help reduce your salt intake without reducing flavour.

8. Reduce your intake of processed grains - eat whole grains in place of processed grains, but limit them to a small part of your diet - use beans and pulses to replace them where possible. Grains are a relatively recent introduction to our diet, and their is a fair bit of evidence to suggest that the side effects of digesting grains is still not well understood.

9. Broaden your diet - try new types of produce, cook them in new ways - the internet will provide you with a hundred recipes for anything you don't already know how to cook.

10. Buy food from farmers, not from scientists or large commercial organisations - we've been farming for 000's of years, and scientifically/commercially dabbling with the food supply on a widespread basis for around a hundred....the scientists and accountants still know a lot less about what we need to eat than the farmers. The rates of obesity and diet related illnesses have sky-rocketed in direct proportion to the popularity of processed food products....they should make up a very small part of your diet.