For you as the payer, the funds are paid (for tax purposes) when they leave your control. For example, if you mail a check on December 29, and there are funds in your account, then it is a December expense even if the payee does not cash the check until January. (However, if you mail the check on the 29th, but the check is post-dated to January, or there are no funds in the account until January and you verbally instruct the payee to wait--or you are taking advantage of the holiday delays to not deposit the funds until January--then the funds did not leave your control until January and are a January expense on your business tax return.
I don't think we can help much with Quickbooks, there is a separate support forum for that. However, I would imagine that if your expense occurs in December, that's when it should be recorded, even if the check was not deposited until later.
