What Vimshottari Dasha is and how long each period lasts
Vimshottari Dasha is Jyotish's way of dividing life into stretches, each with its own ruling planet. The cycle takes 120 years and consists of nine major periods, the mahadashas. The order and lengths are the same for everyone: Ketu — 7 years, Venus — 20, Sun — 6, Moon — 10, Mars — 7, Rahu — 18, Jupiter — 16, Saturn — 19, Mercury — 17. They add up to exactly 120.
Charts differ only in where on the cycle the count starts. After that the chain goes in order without gaps, and after Mercury comes Ketu again. The whole cycle is 120 years, so a single life usually takes up only part of it.
Right away, about the status of the method. The arithmetic of dashas is solid: the Moon's position is calculated astronomically, the rest is addition and multiplication. But that the Saturn period is “about discipline” and the Venus period “about relationships” is the tradition's interpretation, and it has no evidence base. So first the calculation, and at the end a test that may not hold.
How to calculate the first Dasha period from the Moon's nakshatra
The starting point is set by the Moon at the moment of birth. The sidereal zodiac is divided into 27 nakshatras of 13°20′, and the same nine rulers are assigned to them in a circle in the same order: Ashvini — Ketu, Bharani — Venus, Krittika — Sun and so on. At the tenth nakshatra, Magha, the order begins anew, so each planet has three nakshatras: for Ketu these are Ashvini, Magha and Mula.
The ruler of the nakshatra the Moon stands in becomes the ruler of the first period. What matters next is what share of the nakshatra the Moon has already passed: the same share of the first period counts as used up. The formula: balance = length of period × (1 − share passed).
An example. Birth on September 11, 1977, 09:00, Moscow. The Moon is in the nakshatra Ashlesha, the ruler is Mercury, the period is 17 years. Of 13°20′, 11°52′ has been passed, that's 89%. The balance: 17 × 0.1097 ≈ 1.86 years, that is, a year and ten months. Then in order: Ketu from July 1979, Venus from July 1986, Sun from July 2006, Moon from July 2012, Mars from July 2022, Rahu from July 2029.
Inside a mahadasha there are nine sub-periods, the antardashas. The first belongs to the ruler of the mahadasha itself, then the same order. The length of a sub-period is the length of the mahadasha multiplied by the planet's years and divided by 120. The shortest is Sun in Sun: 6 × 6 / 120 = 0.3 years, about 110 days. The longest is Venus in Venus, 3 years 4 months.
Why a Dasha period begins before the date of birth
From the formula follows a thing that in a finished calculation looks like a typo: the first period has a nominal start, and it stands before the date of birth. The period is counted as if the Moon entered the nakshatra on its first day. By the time of birth part of the nakshatra has been passed — which means part of the period passed “before life.” What remains is called the dasha balance.
An example. Birth on March 4, 2020, 15:00, Moscow. The Moon is at 3°21′ of the nakshatra Ardra, the ruler is Rahu, 18 years. 25% of the nakshatra has been passed, so four and a half years of the period fall on the time before birth, and about thirteen and a half remain for life. The nominal dates: August 28, 2015 — August 28, 2033. “The period 2015–2033” for someone born in 2020 is exactly this case, not an error.
Our calculation marks such periods. On the reading page, instead of bare years it says: “from birth to 2033 — the remainder of the period begun in 2015.” If the nominal start fell in the year of birth itself, the tail about “begun in…” is not shown — it would be nonsense. All the following mahadashas begin during life already, and have ordinary “from — to” years.
By how much Dasha dates shift if the birth time is inaccurate
In an hour the Moon passes from 0.49° to 0.64° — from 3.7 to 4.8% of a nakshatra, 4.1% on average. By that same percentage of the first period's length its start shifts. The longer the first period, the larger the shift.
We ran the engine over 20 thousand random moments of birth from 1950 to 2025. The average date shift for an hour of error: first period of Sun — 90 days, Ketu or Mars — 105, Moon — 150, Jupiter — 241, Mercury — 255, Rahu — 271, Saturn — 285, Venus — 300. The extreme values in the sample are 81 and 351 days. On average an hour of error is three to ten months, in the extreme case almost a year.
All the boundaries of life shift at once and by one amount: the chain is rigid, and the error doesn't dissolve with age. A later birth time gives earlier dates. In the 1977 example, 10:00 instead of 09:00 moves each boundary 251 days back: the start of the Mars period — from July 23, 2022 to November 14, 2021. Fifteen minutes there is 63 days, five minutes 21 days.
What this does to the answer “which period is it now.” On a sample of births from 1950–2005, calculated as of September 20, 2026, an hour of error changed the current mahadasha in roughly 5% of cases, and the current sub-period in 34%. With fifteen minutes — 1 and 9%. Without the birth time the dates float by years: for September 11, 1977 the start of the Venus period comes out anywhere from March 1981 (birth at 23:59) to September 1992 (at 00:00), and at noon the Moon is already in Magha and the first period is Ketu, not Mercury. If the time is unknown, the site calculates the chart for noon.
Why Dasha periods don't match on different sites
Besides the birth time there are two decisions a calculator makes silently. The first is the ayanamsha, the correction between the tropical and the sidereal zodiac. Our engine uses Lahiri. The Raman ayanamsha differs from it by almost a degree and a half — that is 11% of a nakshatra, and all dates move by from eight months (the first period of Sun) to over two years (the first period of Venus). Krishnamurti differs from Lahiri by about 0.1°: a shift of 16 to 53 days.
The second is the length of the year. Our calculation takes a year of 365.25 days. Some practitioners count dashas in years of 360 days, and then each boundary comes earlier by 5.25 days for every year from birth: by forty, 210 days accumulate. The tradition has no single answer as to which variant is right — schools differ.
Hence the order for comparing two calculators: first the Moon's nakshatra and the degree in it, then the ayanamsha and the year length. If everything matched and the date didn't, someone has an arithmetic error.
How to test Dashas against your own life
The tradition claims that a change of mahadasha is a change of chapter in life. This can be tested on the past, but only in this order. First, before any calculation, write on paper three turning points — month and year after which life went differently: a move, a change of profession, the start or end of a marriage, a loss. If only the year is remembered, take July 1. Don't change what's written.
Then calculate the current period and reconstruct the boundaries backward, subtracting the lengths of the periods in reverse order, as in the example above; the exact dates of the current period are in the year forecast in the “Jyotish” mode. A hit is when there are no more than 12 months between the event and the nearest boundary. The threshold: two hits out of three.
Where the threshold comes from. Nine boundaries over 120 years with a window of two years each is 18 years out of 120: a random date falls into a window with probability 15%. Two out of three then come out by chance in roughly 6% of cases, at least one in 39%. A simulation on the engine (6 thousand random births, three random dates each) gave 6 and 37%. One hit is background, not a signal. If short periods fell on your life, the background is higher: your own share = the number of boundaries on the stretch the events were drawn from × 2 / the length of the stretch in years. In the 1977 example, for the whole life there are 5 boundaries over 49 years, 20%, and two out of three will already come up by chance in 11% of cases.
A control: shift all the boundaries three years forward and count the hits again. The shortest period is six years, so the false windows don't overlap the real ones. False boundaries hit as well as real ones — which means the match is accidental. The condition of applicability is a birth time accurate to a quarter of an hour: with an error of an hour the boundaries float by a span comparable to the width of the window. Fewer than two out of three — on this biography the dashas did not show themselves, and there's no need to fit the wording to the result.