I'm not sure I agree with GMK. I think it is perfectly legitimate (and in fact commonplace) to use "cash basis" accounting for determining which pay is deferral. If the "pay date" (the date on which the participant has an irrevocable right to cash) occurs after the entry date, but part of the pay is for work performed before the entry date, the entire amount of that pay is "deferrable."
Sorry, but I have no authority (at my fingertips), but the "pay date" is the first day on which the participant has the right to the cash, and hence can defer it at that point if he or she is otherwise eligible for participation.