IRA Distribution–IRS Tax Year Rule

Looking for the IRS definition as to when an IRA distribution is considered as made for tax year purposes. I have from a reliable source that, once funds are withdrawn and made unavailable (e.g., on Dec 31), that date is considered a distribution for tax purposes.
But, what if a brokerage waits a few days, does their processing, and then decides to reckon the tax year as that processing date? Seems to me that would violate IRS rules.
Wondering if the IRC or regs explicitly cover this.