Obligation of amounts
Except as provided in paragraph (4) and subject to paragraph (2), a public housing agency shall obligate any assistance received under this section not later than 24 months after, as applicable—
(A) the date on which the funds become available to the agency for obligation in the case of modernization; or
(B) the date on which the agency accumulates adequate funds to undertake modernization, substantial rehabilitation, or new construction of units.