A Solar-Referenced 13-Cycle Civil Calendar-Astronomical Basis, Mathematical Structure, and Civil Implementation Model

This working paper presents a solar-referenced 13-cycle civil calendar architecture designed to align annual timekeeping directly with the tropical Earth–Sun cycle. The proposed model divides the year into thirteen equal 28-day months (364 structured days), supplemented by intercalary day placement to maintain long-term astronomical accuracy. The calendar dynamically anchors its annual reset to the March equinox (UTC), establishing an observable astronomical reference point independent of inherited civil date conventions. By adopting the Gregorian leap-year correction rule, the model preserves equivalent long-term drift performance (~1 day in approximately 3,300 years) while reorganizing internal structure to achieve complete weekly symmetry (52 full weeks), fixed month lengths, and permanent weekday-date alignment. The paper includes mathematical formulation, deterministic Gregorian conversion methodology, comparative analysis with the Gregorian and International Fixed Calendar systems, and discussion of civil implementation considerations. This document is presented as a structural timekeeping model for evaluation and research. It does not incorporate predictive, astrological, or interpretive systems. Its objective is to examine whether equinox-anchored perennial symmetry offers administrative and structural advantages over irregular month-based civil calendars.