Term of auditors for rotation

Hi. I had a query relating to rotation of auditors. If an auditor firm has been appointed in a pvt ltd Co for 2nd term and then in 2nd year of the 2nd term the Co comes under the applicability of sec 139(2) then how are we going to calculate the total term of the auditor. Will previous term be taken into account in this situation or the auditor can be appointed for one more term?