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Argument-fuelling reference
Brihat Parashara Hora Shastra prescribes a 120-year Vimshottari cycle but is silent on the exact length of a "year." Three conventions are in use today and each gives slightly different Mahadasha end dates. Over the full cycle, the spread is days for the inner planets and weeks for Saturn. Here is what each convention is, what it gives in practice, and which one we use.
Each convention defines "one year" differently. The drift column shows how much each convention's Saturn Mahadasha (19 years) lags or leads behind the 365.25-day baseline over its full duration.
BPHS (chapters 46-51) lays out the Vimshottari scheme — nine planets, 120-year cycle, fixed proportions (Ketu 7y, Venus 20y, Sun 6y, Moon 10y, Mars 7y, Rahu 18y, Jupiter 16y, Saturn 19y, Mercury 17y). What the text never specifies is whether "one year" is reckoned in Julian days (365.25), Savana civil days (360), or any of the modern alternatives. Different teachers and software packages picked their convention based on their teacher's teacher's habit; the choice has been opaque to end-users for decades.
In practice, the difference between Julian (365.25) and Gregorian (365.2425) is functionally invisible — about 3.5 hours over a 19-year Saturn Mahadasha. The difference between Julian and Savana is more visible: the Saturn Mahadasha under Savana ends ~100 days earlier than under Julian. A reader who expects Saturn Mahadasha to end in January 2030 under one tool will see it ending in October 2029 under a Savana tool. That is a real, observable difference.
It is the mainstream convention. JHora — the de-facto reference Jyotish software for the past two decades — defaults to 365.25. Parashara's Light, KP Astro, and most published dasha tables in books like Sanjay Rath's or K.N. Rao's use 365.25. Choosing it as our default means our published dates align with what most users will see in their software, their books, and their teacher's example charts. When ours and another tool's dasha-end-dates differ by hours, that is round-off noise; differences of weeks indicate the other tool is on a 360-day Savana convention, which we document in the Calculation Standards.
We use exact millisecond arithmetic, not month-truncation: `new Date(baseDate.getTime() + years × 365.25 × 86400 × 1000)`. Many older calculators add years by month-by-month increments which truncates fractional days and drifts by hours over each sub-period. Across a 120-year Vimshottari cycle, that accumulates to days. Our published Antardasha and Pratyantar dates therefore agree with reference implementations to within the round-off limit of the underlying timestamp.
Suppose Saturn Mahadasha begins on 2026-01-01 00:00 UT. Under each convention, when does it end?
Julian (365.25): 2045-01-01 18:00 UT (6939.75 days). Gregorian (365.2425): 2045-01-01 14:35 UT. Sidereal (365.2563627): 2045-01-01 20:53 UT. Savana (360): 2044-09-23 00:00 UT — three months early. A user who switches between an engine on 365.25 and one on 360 will see the end-date differ by 100 days — large enough to lose money on date-bound decisions.