Hello, I'm Dr. Deb and will do my best to help you today.
I've frequently used these products even though they've been applied after the expiration date on the package. As you noted, they may not be quite as effective as if they were used before the indicated date but they certainly will do no harm. In some instances, I'm fairly certain that there was no loss of efficacy...although it's sometimes difficult to know for sure.
If I were in your shoes, I'd apply the product you have and not worry that my cat would suffer any adverse reactions by doing so.
I hope this helps. Deb