How the first mahadasha start date is set
The first mahadasha starts at birth, but only its balance remains. Its start is therefore the birth date, and its end is the birth date plus the balance in years.
What it is
The balance is the lord's years times the fraction of the birth nakshatra still to be crossed, as set out in the dasha balance article. The start date of later periods follows from the end of this one.
How it is worked out or read
The tool gives the start and end dates on the timeline. Because the first period is partial, its shown start is the birth date and its length is the balance.
Where it appears on this site
The first start is the birth
The first mahadasha starts at birth, but only its balance is still to run. For the purposes of the timeline, its start is therefore the birth date, and its end is the birth date plus the balance in years. The reader sees a period that appears to begin on the day of birth and ends sooner than its full length, which is the expected result of a partial first period. It is important to remember this when comparing with a dasha that starts at a later date in another chart.
Because the first period is partial, the timeline's first full period is the second one, which begins at the end of the balance. This is the point at which the timeline first shows a full-length mahadasha, and a reader can use it as the first complete reference period for a life.
Later mahadashas and their starts
Each later mahadasha starts exactly at the end of the one before it. The start date is therefore the sum of the balance and the full lengths of all earlier periods, added to the birth date. A reader can check any start by adding the lengths of the earlier periods to the birth date plus the balance, using the 365.25-day year. The tool shows the dates, and the arithmetic can be repeated by hand for any period.
Because the start dates depend on the birth date and time, the reader should confirm the birth details before relying on a specific start. A change of an hour in the birth time may change the balance by a small amount, and that change is then carried forward into every later start date.
Using the start dates
Start dates are useful for seeing when a major theme began, or for comparing the timelines of two people. The reader should note that the start of a mahadasha marks the beginning of a graha's rule, not an event. When a start date is compared with a life event, the comparison is an observation, and the reader should avoid treating the match as proof of a cause.
Checking a start date against the birth time
A start date depends on the birth time, and it is worth checking how sensitive it is. Move the birth time forward and backward by a few minutes and see whether the first period's end date changes, and by how much. If the Moon is far from a boundary, the change will be small. If it is close, the change may be large enough to alter the start of the second period. Knowing this in advance tells the reader how far the timeline can be trusted for a given birth.
The check is quick with the tool and gives a clear sense of the margin of uncertainty. A reader can then state the dates with an honest range where the birth time is in doubt, rather than as a single exact day.