29 C.F.R. § 4041.25

(a) Notice requirement. The plan administrator must file with PBGC a standard termination notice, consisting of PBGC Form 500, completed in accordance with the instructions thereto, on or before the earlier of—

(b) Change of proposed termination date. The plan administrator may, in the standard termination notice, select a proposed termination date that is later than the date specified in the notice of intent to terminate, provided it is not later than 90 days after the earliest date on which a notice of intent to terminate was issued to any affected party.

(c) Request for IRS determination letter. To qualify for the distribution deadline in § 4041.28(a)(1)(ii), the plan administrator must submit to the IRS a valid request for a determination of the plan's qualification status upon termination (“determination letter”) by the time the standard termination notice is filed.