One reason to have a non-calendar year health plan is to avoid the crowd that renews on January 1 and the associated delays in getting responses, etc., because everyone is asking about their renewals at that time.
Unless you have anything to show otherwise, it appears to me that "prompt pay" laws are at the state level.
You will need to review applicable state law as well as federal district court cases such as discussed in this article: http://www.healthcarepayernews.com/content/ama-erisa-doesnt-preempt-state-payment-regulation#.U_ZOgE90zcs