How the Assamese Calendar Is Calculated: Panjika Tradition, Surya-Siddhanta Astronomy, and a Verified Computational Reconstruction

The Assamese calendar (অসমীয়া বৰ্ষপঞ্জী) is a sidereal solar calendar whose twelve months correspond to the Sun's transits of the twelve nirayana zodiac signs, with a lunisolar layer of tithis and lunar months superimposed upon it to fix religious observances. Its computation has traditionally been the tacit craft of a small number of almanac (panjika) compilers, transmitted through printed editions rather than through published method statements. This paper sets out the full derivation in reproducible form: the textual and astronomical basis in the classical Surya Siddhanta, the arithmetic of mean and true longitude, the rules governing month boundaries, tithi, intercalary months and festival assignment, and the era systems printed alongside. A reference implementation computing the calendar from first principles, without lookup tables, is described and verified against a printed Assamese panjika across the interval 1950–2100 (73,204 days; approximately 1,812 month boundaries), reproducing solar month starts with a solar-longitude RMS of about three arcseconds and a single disputed boundary. Limitations of the classical lunar model are quantified, and three unresolved questions — the precise month-start convention, an anomalous lunar phase offset, and the chronology underlying the Bhaskarabda era — are stated explicitly rather than concealed. The aim is to place a living calendrical tradition on a documented, testable and durable footing. Reference implementation: the calendar described here is freely available to the public at oxomiacalendar.in, with an expanded discussion at oxomiacalendar.in/how-the-assamese-calendar-works.html. It requires no account, no payment and no server: the computation runs entirely in the reader's browser, and it carries no advertising. Every parameter needed to reproduce the results is listed in the paper, so the findings may be verified independently of that implementation.