theyre being timey wimey with how much time has passed so we dont know how much time has passed yet (this is mostly to do with the real time affects on riggs age filming the show) we know that ATLEAST a year or 2 has passed but theyve stated that atleast for now theyre not going to mention how much time has passed.
i like how they actually mention this in the show last man on earth. i dont know if anyones ever asked kirkman about this or if hes even aware such a thing would happen but it would be cool for a future storyline