Dear, the expiration of contacts is different from the expiration of your eyeglasses prescriptions. They are different things while they have some sort of relations. We must buy contacts or glasses according to the prescriptions. If the prescription is expired, that means your eyesight has changed. You need to correct your vision problems by changing another pair of contacts or glasses though the contacts you bought are within the expired date. You can have your eyesight checked to see whether the prescription changed or not.
I think eyeglass prescriptions do not expire only if you take good care of your eyes and protect your eyes in your daily life. My brother and I both have the mild myopia. We adapt many ways to protect our eyesight. We keep a balanced diet and do eye exercise regularly. But if you can't do this, you'd better get your eyes checked each year to make sure that your glasses are still effective.