Dry spices loose their strength rapidly, with the exception of things like whole peppercorns, whole nutmeg, etc.
Does your store have a bulk foods section? Most sell spices.
Also, a lot of health food stores carry a good selection of bulk spices.
I keep a very limited selection on hand at home, and buy just what I need, when I need it.
Keep on hand only the few things you use regularly.
It takes some prior planning, but you will always have fresh spices.
In addition, that can of mustard might have been $5, you used .25 worth in your last recipe and are now throwing the rest away.