38 CFR 17.129: Retroactive payments prohibited.

When a claim for payment or reimbursement of expenses of services not previously authorized has not been timely filed in accordance with the provisions of § 17.126, the expenses of any such care or services rendered prior to the date of filing the claim shall not be paid or reimbursed. In no event will a bill or claim be paid or allowed for any care or services rendered prior to the effective date of any law, or amendment to the law, under which eligibility for the medical services at Department of Veterans Affairs expense has been established.