Jyothshi · Vimshottari
Every mahadasha divides into nine antardashas — sub-periods, one for each planet, in a fixed order and fixed proportions. If you know which mahadasha you are in and roughly when it began, the nine dates follow. No birth time is needed, because the birth moment is only used to decide which mahadasha is running, not how it divides.
Each antardasha takes the same share of the mahadasha as its own lord's term takes of the 120-year Vimshottari cycle, running in this fixed order from the mahadasha's own lord.
Deciding which mahadasha you are in requires the Moon's exact position at birth, which requires an exact birth time. Dividing that mahadasha into its nine antardashas requires neither — only the lord and the start. So if someone has already told you your mahadasha, the sub-periods are available to you today. Adding your birth details later replaces the estimate with exact dates.
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