I say the "90 days" begins the day after the payment is due, i.e., the first day after the due date is day #1. In your case this would be the day after the due date with the due date being 30 days after the date posted on the billing document. 

Presumably your Association either has bylaws or the wording you mention is, or has been, established by affirmative majority vote at an Annual Meeting. In my case our Association voted several years ago to set the due date as the next January 1st. That determination has been continued at subsequent Annual Meetings which are held in July and is posted in the Minutes. Even with nearly six months notice we still have a few "stragglers" but, with firm notice at each Annual Meeting that collection action will begin, we currently have 100% paid-in-full.

I also would be interested in other interpretations.

There are 32 property owner members is my road association. The road association (Whittemore Cove Road Association) was first formed as a nonprofit corporation with Articles of Incorporation filed in August 1991. It also formed under Title 23 in 2001.