Vimshottari is the dasha system in general use, and dashas are the feature that most sharply distinguishes Jyotish from Western astrology: a built-in scheme for saying when, derived from the birth chart itself rather than from current planetary positions.
The cycle runs 120 years, divided among the nine grahas in fixed but unequal shares:
The order is fixed — Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury — and repeats. What is not fixed is where you enter it. That is set by the Moon: each nakshatra is assigned one of the nine as its lord, and the proportion of the nakshatra the Moon has already traversed at birth is the proportion of that mahadasha already spent. Someone born with the Moon three-quarters of the way through a Venus nakshatra begins life with five years of Venus remaining out of twenty.
Each mahadasha subdivides into nine antardashas in the same proportions, and those subdivide again, as far as practitioners care to go.
The consequence worth understanding: your whole life timeline is computed from one number — the Moon's longitude — and that number depends on birth time. A one-hour error moves the Moon about half a degree, which is 4% of a nakshatra, which on a twenty-year Venus period is roughly nine months. Dasha predictions inherit the uncertainty of the birth time and cannot be more precise than it.