While generally a date in a contract, such as an expiration date on a  counteroffer, is enforceable, courts will not enforce a date that as clearly a typo or other plain error. In the context of selling a home, it would be very unusual for a seller to keep a counter open for a whole month and refuse any other offers that came in during that time; therefore, it is very plausible that it was meant to be a 3 day time period, and a court would likely enforce 3, not 30 days, against the seller. That being the case, if you accepted the offer within 3 days, the would contractually have to take it. After 3 days, they could accept other offers.