It doesnâ€™t matter. Everyone is right in that it you should have put a clause in your Counter that the expiration date of the Offer was extended to the date you presented your Counter. Nevertheless, if you failed to do so, a strong argument can be made that the defect was corrected and/or a new revived contract was created when your Counter was accepted.
HOWEVER, the reason none of this matters is that there is no such thing as a binding purchase agreement against the BUYER. Offers are full of contingency periods during which buyers can always cancel. Plus the law gives Buyers additional rights to cancel depending when and even how some disclosures are delivered even if there are NO contingencies in the Offer itself.
Since you countered to an Offer I presume you are the Seller. The question becomes more complicated if you are now asking because the Buyer is trying to enforce terms of the expired Offer against YOU, which you are now trying to argue do not apply. As always, consult an attorney.