There's two different date settings you may have used:
One is an account-wide setting for all your products, BEFORE which no products can be purchased; to view or edit this setting, go to Seller Admin > Account Preferences > "Make all items unavailable for purchase until" -- just delete the date entered there if you want to remove the block on purchases. Click Submit to apply any changes you make here.
The other is a product-specific setting, AFTER which that product cannot be purchased; to view or edit this, go to Seller Admin > View/Edit/Delete Products, select the relevant product and click Edit, then click Next until you find the Item Expiration field -- just delete the date entered there to disable the expiration. Click Next until you can click Submit to apply changes.