Bird, you are on the money. When they do the corporate audit, a portion of that pay-period gets accrued as a 2013 payroll expense. Why? Because the service rendered is attributable to 2013.
But the key here is that it is a non-event until the amount is WITHHELD. That's the obvious point for which no reg is needed. If not WITHHELD until 2014, it is a 2014 contribution. It is not due to the Plan until it is WITHHELD (one h or two?).