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Bags of compost are not marked with a sale by date,but do the nutrients degrade over time.  Some bags are marked telling you the number of weeks the fertilizer will sustain your plants, but that's about all.  Any info on this?

It depends, if it has been stored under cover then it can last up to 18 month. If you have the chance of getting it cheap you can always add some Fish Blood and Bone to boost it.