The DOL reg allowing for limited scope audits references that timing. 29 CFR 2520.103-5 - Transmittal and certification of information to plan administrator for annual reporting purposes.§ 2520.103-5 Transmittal and certification of information to plan administrator for annual reporting purposes.
(a) General. In accordance with section 103(a)(2) of the Act, an insurance carrier or other organization which provides benefits under the plan or holds plan assets, a bank or similar institution which holds plan assets, or a plan sponsor shall transmit and certifty such information as needed by the administrator to file the annual report under section 104(a)(1) of the Act and § 2520.104a-5 or § 2520.104a-6:
(1) Within 9 months after the close of the plan year which begins in 1975 or September 30, 1976, whichever is later, and
(2) Within 120 days after the close of any plan year which begins after December 31, 1975.