If the marking says "Use by...", the product's safety is guaranteed only up to that date. If it says "Best if used by..." the manufacturer is recommending that it be used by then, for optimum quality.
Most canned and jar items are good for a while after their expiration date. But it's not wise to take it to extremes. Many factors, like exposure to light, temperature variations and chemicals in the packaging can adversely effect the contents. When it doubt, throw it out.
