expires_at method

before_save :set_expiration_date #once its been saved update the expires_at attribute with the set_expiration_method

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

def set_expiration_date
  self.update_attribues(:expires_at => created_at + 6.months)
end