47 C.F.R. § 400.10 – Closeout.

(a) Expiration of the right to incur costs. The right to incur costs under this part will expire as of the end of the period of performance. The grant recipient and its subrecipients and contractors may not incur costs for Federal reimbursement past the expiration date.
(b) Final submissions. Within 90 days after the completion of projects and activities funded under this part, but in no event later than the expiration date identified in paragraph (a) of this section, each grant recipient must submit—
1. (1) A final voucher for the costs incurred. The final voucher constitutes the final financial reconciliation for the grant award.
2. (2) A final report to NHTSA, following the procedures of 2 CFR 200.343(a).
(c) Disposition of unexpended balances. Any funds that remain unexpended after closeout shall cease to be available to the recipient and shall be returned to the government.