How is the starting dasha decided?

By the nakshatra the Moon occupied at birth. Each nakshatra has a fixed Vimshottari lord, and that lord's mahadasha is the one running at birth. The portion of the nakshatra the Moon had already crossed determines how much of that period had already elapsed.

Are there other dasha systems?

Yes — Ashtottari, Yogini, Chara dasha and several others, each with its own applicability conditions. Vimshottari is the default in Parashari astrology and is the system VedikPatrika uses. Mixing systems without stating which is being used is a common way for readings to become unfalsifiable.

Does a bad mahadasha mean bad years ahead?

No planet's period is inherently bad. A dasha delivers what that planet rules in your specific chart, according to its strength and placement. The same Saturn mahadasha that is heavy in one chart is the making of another. Any reading that labels a period good or bad without reference to the chart is not reading the chart.

Vimshottari Dasha: The Timeline of a Kundli

विंशोत्तरी दशा

What is Vimshottari dasha and how is it calculated?

Vimshottari dasha is the 120-year planetary period system that gives a Vedic chart its timeline. It is calculated from the nakshatra the Moon occupied at birth: that nakshatra's lord is the mahadasha running at birth, and the eight remaining periods follow in fixed order. It is what tells a reading when something is likely, not merely whether it is possible.

The 120-year cycle

Each of the nine grahas holds a mahadasha of fixed length, and the nine lengths sum to exactly 120 years. Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19, Mercury 17. The order never varies; only the starting point does.

That starting point comes from the Moon. The 27 nakshatras are assigned the nine lords three times over in the same repeating order, so whichever nakshatra the Moon occupied at birth names the mahadasha you were born into. How far the Moon had travelled through that nakshatra determines how much of the period was already spent — the balance of dasha at birth.

Mahadasha, antardasha, pratyantardasha

Each mahadasha subdivides into nine antardashas (bhuktis) in the same fixed order, each proportional in length to its own lord's share of the 120 years. A Venus mahadasha of 20 years opens with a Venus-Venus antardasha of about 3 years 4 months, then Venus-Sun of a year, and so on. Each antardasha subdivides again into pratyantardashas.

In practice the antardasha frequently matters more than the mahadasha. A nineteen-year Saturn period is far too long to have a single character, and what actually distinguishes one stretch of it from another is which antardasha is running and how those two lords relate in the natal chart.

How a dasha is actually read

The question is never 'is this a good dasha'. It is: what does this planet rule in this chart, how strong is it, where does it sit, and what does the antardasha lord's relationship to it look like. Saturn's dasha for an Aquarius ascendant — where Saturn is the lagna lord — is a completely different proposition from Saturn's dasha for a Cancer ascendant, where Saturn rules the 7th and 8th.

Transits are read against the dasha, not instead of it. The standard approach is that the dasha says what is available and the transit says when it surfaces. This is why a report that gives you date ranges should be deriving them from dasha boundaries and transit ingresses together, and should be able to tell you which.

Why the birth time matters here

The Moon covers a nakshatra in roughly 24 hours, so a birth time wrong by a few hours can shift the balance of dasha at birth by months, and a birth time wrong by most of a day can change the starting mahadasha entirely. The dasha timeline is the part of a chart most sensitive to birth time after the ascendant itself.

Where the time is approximate, dasha boundaries should be presented as approximate too. A report that gives you a precise date from an uncertain birth time is claiming precision it does not have.

The 120-year cycle in full

The nine mahadashas always run in this order, and their lengths always sum to exactly 120 years. Only the starting point varies, and that is set by the nakshatra the Moon occupied at birth.

Vimshottari mahadasha order and duration in years
#	Mahadasha lord	Years	Cumulative
1	Ketu	7	7
2	Venus	20	27
3	Sun	6	33
4	Moon	10	43
5	Mars	7	50
6	Rahu	18	68
7	Jupiter	16	84
8	Saturn	19	103
9	Mercury	17	120
	Total	120

Common questions

How is the starting dasha decided?: By the nakshatra the Moon occupied at birth. Each nakshatra has a fixed Vimshottari lord, and that lord's mahadasha is the one running at birth. The portion of the nakshatra the Moon had already crossed determines how much of that period had already elapsed.
Are there other dasha systems?: Yes — Ashtottari, Yogini, Chara dasha and several others, each with its own applicability conditions. Vimshottari is the default in Parashari astrology and is the system VedikPatrika uses. Mixing systems without stating which is being used is a common way for readings to become unfalsifiable.
Does a bad mahadasha mean bad years ahead?: No planet's period is inherently bad. A dasha delivers what that planet rules in your specific chart, according to its strength and placement. The same Saturn mahadasha that is heavy in one chart is the making of another. Any reading that labels a period good or bad without reference to the chart is not reading the chart.