Personally, I do not think it is open to interpretation. That is why I have seen documents that specifically mention what pay period is used. If not mentioned then it has to be any pay remitted to the employee while a participant (if not using full year pay). It is not relevant when it was 'earned', only when 'paid'. If a person enters the plan on Oct 1, and Oct 1 is a paydate, then they should be deferring on that pay. Otherwise you have a failure to allow deferrals and an EPCRS incident.
Strong opinion to follow.