Aloha Airlines is required by law to have its aircraft engines tested and recertified after 5,000 flight hours. Molokai Maintenance performs the engine tests and recertification for $2,200 per aircraft. For financial accounting purposes, Aloha establishes a reserve account and accrues a maintenance expense of 44 cents per flight hour. When the maintenance is done, the amount paid for the maintenance is deducted from the reserve account. For tax purposes, when can the maintenance expense be deducted?