25 C.F.R. § 1000.1820
When will the retrocession become effective?
The retrocession becomes effective on the date that is mutually agreed to by the parties in writing. In the absence of a mutually agreed upon effective date, the retrocession becomes effective on the earlier of:
(a) One year after the date the Tribe/Consortium submits its notice of retrocession; or
(b) The date the funding agreement expires.