Tar does not expire like food, but it can change in storage. It may thicken, separate, form a skin, lose lighter fractions, or become harder to apply.
A tin of tar is not usually ruined by a printed date in the way milk or medicine might be. Many tar products are heavy, stable mixtures that can sit for a long time. That said, storage still matters. Air, heat, cold, contamination, and loose lids can change the texture and usefulness of the material.
Wood tar may become thicker as lighter volatile parts slowly leave the mixture. A skin can form on top. Settling can occur if the product contains different fractions. Some modern products also contain solvents, oils, or additives that change the shelf-life question completely.
The important distinction is between chemical danger and practical quality. Old tar may still be recognisably tar, but it might brush badly, dry unevenly, smell stale, contain grit, or fail to behave as the maker intended. For heritage work, that matters because a poor coat can create a patchy or sticky finish.
The best general rule is to store tar sealed, cool, and away from flame, then judge it by the manufacturer’s instructions and by appearance, smell, and consistency. If a product has separated badly or no longer matches its intended use, it should not be forced into a serious project.