Vimshottari dasha explained, and how every sub-period is dated

Vimshottari is the timing system most readings stand on. The arithmetic is simple and unforgiving: nine lords, 120 years, and a starting point fixed by the Moon.

Nine lords, 120 years

The sum is 120, the traditional full span of life. The sequence repeats, so a native (the person whose chart is being read) who starts in Saturn goes on to Mercury, then Ketu, and so on.

Where a life begins in the cycle

The Moon's nakshatra at birth names the first mahadasha. How far the Moon has travelled through that nakshatra says how much of the period has already elapsed. A Moon one quarter of the way through Rohini starts the native in a Moon mahadasha with a quarter of its ten years spent: seven and a half years remain. That fraction is the whole game: a Moon longitude wrong by one degree moves every subsequent boundary by about 274 days, roughly nine months, in a Moon period and by more in a longer one.

Bodhlok takes the Moon from the Swiss Ephemeris at the birth instant, in the ayanamsa of your calculation profile, and carries the fraction through at full precision. The year used for the cycle is the canonical 365.25-day year, so the periods neither drift nor duplicate across the long sequence.

Sub-periods down to the day

Each mahadasha is divided among the nine lords in the same proportions, starting from the mahadasha lord: the Venus antardasha inside a Saturn mahadasha lasts 19 × 20 / 120 = 3 years and 2 months. Each antardasha is divided again into pratyantars by the same rule. The desk shows the current mahadasha, antardasha and pratyantar on every screen, and the full table with dates, so a client's question can be placed inside a window that has a start day and an end day.

Reading a period

A mahadasha lord's results come from its house lordship, placement, dignity and the houses it aspects; the antardasha lord modifies them, and the relationship between the two lords (friend, enemy, conjunction, mutual aspect) decides whether the period runs smoothly. The consult reads these from the chart when it answers a timing question and names the sub-period it is relying on, so you can disagree with it.

Ashtottari, Yogini and Chara

Bodhlok also runs Ashtottari (108 years, eight lords, applied by the classical condition on Rahu's placement), Yogini (36 years, eight yoginis) and Jaimini's Chara dasha (sign-based, with the co-lord rule for Scorpio and Aquarius). They are on the same desk so a date can be cross-checked rather than trusted to one system.