In the United States, the bright yellow, mild flavored mustard tends to be the most widely used. When stored unopened on a shelf, not in extreme heat or cold, this mustard can last one to two years past the date on the label or bottle, depending on whether that date refers to when it is best used by or its expiration. Dijon mustard varieties can last longer because they have a bit more bite to them, with a higher proportion of vinegar and other spices than the milder yellow mustard has. These mustard types can last up to three years.

Once a jar or bottle of mustard has been opened, its useful life decreases because it is no longer sealed against environmental contaminates. However, with the vinegar content, even opened mustard can last up to a year past its date, as long as it's kept in the refrigerator and you close it properly after each use. If the mustard is used infrequently, a little bit of its liquid may separate from sitting undisturbed for long periods. Stirring or shaking before use fixes that easily.

To make sure that mustard lasts as long as it should, use correct food-handling techniques. Don't introduce crumbs into a jar of mustard while making sandwiches. Those crumbs of bread, cheese or meat could mold and spoil the whole jar of mustard. Wipe the neck of the jar and the top to prevent the mustard from clumping and drying as the vinegar – and its protection – dissipates, leaving that dried mustard vulnerable to spoiling, then contaminating the whole jar or bottle.

The bottom line on how far past the expiration date mustard can be considered safe to use lies in its appearance and smell. If a lot of the liquid has separated, leaving the ground mustard seeds and other spices in a dry clump, or it has an off odor, the best bet is to throw it out. If it looks and smells fine, you can probably use it without concern.