How are you determining a customer's eligibility for benefits, such as a higher APY, or some other reward, if they conduct at least X number of debit card transactions in a statement cycle: based on the date of the transaction or based on the posting/crediting date? I don't have the source but my recollection is that the regulators think the date of the transaction is the clearest and most fair to the consumer, since that is how the customer will know whether they've reached the minimum number to enjoy the benefits.

It really doesn't matter. What matters is that you are accurate and you fully and clearly explain the requirements to the consumer. How you will know that they conducted an actual transaction before you receive it in order to post it to the account - I will leave that up to you to figure out as to how you are going to count those transactions in any specific statement cycle..