expires_at method

after_save :expires_at #once its been saved update the expires_at attribute

def expired?
    if accepted! #if an offer has been accepted it cannot expire
      return false
    else
      created_at < expires_at ? false : true #if created_at is less than expires_at return false, else return true
    end
end

def expires_at
  created_at + 6.months
end