What a planetary period is
The 120-year planetary period cycle is the most widely used period system in traditional Indian astrology. Life is modelled as a 120-year cycle in which nine planets take turns ruling — each turn is a major period.
The cycle begins from the lunar mansion the moon occupied at your birth. If you were born mid-lunar mansion, part of the first period is considered to have already elapsed, which is why the first major period can look short.
The nine periods
Ketu 7 years, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19 and Mercury 17 — the total is 120 years, and Venus's 20 years is the longest of the nine.
Each major period is divided into nine sub-periods. So even inside a 19-year Saturn major period there are months where the Sun or Mercury rules.
Down to sub-periods
On any given date two planets are running together — major period and sub-period. That is the pair you see in the report, for example 'Saturn major period · Venus sub-period'.
Sub-periods can be divided further (second-level sub-periods, down to days), but our report stays with major period, sub-period and transits — claiming finer precision would be dishonest.
How time windows are produced
Each question has a fixed rule set: which major period, which sub-period and which transit is considered relevant to it. Periods that satisfy those conditions become a 'time window' with a start and an end date.
That is why the same birth details always produce the same result — the rules are fixed and the dates come from the calculation. But remember: this is a traditional system, not a guarantee.