No, it can't be guessed at. At least, you cannot say that's what you're doing. Whatever date you put on the receipt is the date by which the funds must be credited to the recipient's account or be capable of being retrieved by the recipient.
Best advice is to eliminate as must risk as possible and push the date out as far as you think is necessary to create really good odds that the funds will be available by that date, and add the wording to say that they may be available sooner.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8