Defer TT start date before offer
It's normal and good to tell places you're interviewing about any deadlines you face, that way they have the opportunity to move up their decision process in response.  If I were you I would tell the places you're interviewing something like 


  I have a deadline to decide on a postdoc offer at location X by date Y.  If I accept their offer I would still be very interested in taking a job at your school with startdate of year+1.