Each package of Cake Yeast is stamped with a 'Best if Used by' date.

Example of code: March 22 2009

Cake Yeast is very perishable, requiring constant refrigeration below 45°F to maintain its freshness and activity. Always wrap remaining cake yeast tightly and store in an airtight container in the refrigerator. We do not recommend freezing cake yeast.

Yeast is a living organism, and will lose activity over time.  If you are unsure of your yeast's activity, perform the Yeast Freshness Test before using.