47 C.F.R. § 302.10 – Closeout.

(a) Expiration of the right to incur costs. The right to incur programmatic costs under this part will expire at the end of the period of performance. The right to incur closeout costs under this part will expire at the end of the 120-day closeout period, unless this period is extended in writing by the Grants Officer.
(b) Final submissions. Within 120 calendar days after the completion of projects and activities funded under this part, but in no event later than the closeout period expiration date identified in paragraph (a) of this section, each grant recipient must submit—
1. (1) A final financial report to NTIA/NIST, following the procedures of 2 CFR 200.344(a); and
2. (2) A final performance report to NTIA/NIST, following the procedures of 2 CFR 200.344(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 Federal Government.