The system divides a notional human span of a hundred and twenty years among the nine grahas in a fixed sequence with fixed lengths: Ketu seven years, Venus twenty, the Sun six, the Moon ten, Mars seven, Rahu eighteen, Jupiter sixteen, Saturn nineteen and Mercury seventeen. The nine add to a hundred and twenty exactly, and the order never varies. What varies is where in the cycle a life begins.
That entry point is set by the Moon. The twenty seven nakshatras, the lunar mansions the ecliptic is divided into, are assigned to the nine planets three times over in the same order, so the mansion the Moon occupies at birth names the ruler of the first period, and the Moon's position within that mansion fixes how much of the period has already elapsed. Two charts with every planet in the same degree but the Moon a fraction of a mansion apart can therefore run entirely different timelines.
Each major period is subdivided into nine sub periods in the same proportions and the same order, and the subdivision can be carried further, three, four or five levels deep. Readings taken at that depth resolve to weeks. The name comes from the Sanskrit for a hundred and twenty, which is the one thing about the system that is not open to variation.