Why dasha periods do not overlap
Mahadashas do not overlap: each one begins exactly where the previous one ends. The sequence is continuous, and only the first period is partly run at birth.
What it is
Antardashas inside a mahadasha also do not overlap, and they fill the mahadasha in order from its own lord. The arithmetic makes the boundaries meet exactly.
How it is worked out or read
If a timeline seems to show overlap, check whether a period from a different level, such as a pratyantardasha, is being read as a mahadasha.
Where it appears on this site
See the mahadasha and antardasha article.
Why the periods line up without gaps
Mahadashas do not overlap and do not leave gaps, because each one is defined as starting at the exact moment the previous one ends. The timeline is therefore a continuous chain from birth, and every moment in a person's life belongs to exactly one mahadasha and one antardasha. This is part of what makes the system easy to check: there is always a clear answer to the question of which period is running.
The same holds for antardashas inside a mahadasha. Their lengths add up exactly to the mahadasha's length, and the sequence starts with the mahadasha's own lord. A reader who finds two periods that appear to overlap should check whether one of them is a different level, such as a pratyantardasha, or whether a date has been entered in the wrong place.
What an apparent overlap usually means
Apparent overlaps almost always come from one of three sources. The first is reading a sub-period as a main period, which happens when a table omits the level headings. The second is comparing dates from two calendars, civil and panchang, which can differ at the edges by a day. The third is a difference in the birth time that moves the first boundary, which shifts the whole later sequence by the same amount.
When a reader sees such an overlap, the first step is to identify the level of each row, then the calendar convention used, then the birth time. Most apparent overlaps disappear at this stage.
Using the continuous timeline
Because the timeline is continuous, a reader can find the active period for any date without ambiguity. The reader simply moves along the chain until the date falls within a period. This continuity is one of the features that make the Vimshottari system a convenient reference, and it is one reason the site presents it as a single ordered sequence.
Overlap and the question of who decides
Some readers ask whether one dasha can override another when both are active. In the Vimshottari method they are not competing in that way. Both are simply in effect at once, one as the main period and the other as the sub-period beneath it. The question of emphasis is answered by the reader's judgement and by the traditions followed, not by the calendar. A timeline shows which combination is active, and how strongly each graha's theme is expressed is a matter for the chart as a whole.
A reader who wants a simple summary of an overlapping period can describe the mahadasha as the frame and the antardasha as the colour within that frame. This keeps the two levels in their proper relation and avoids treating either one as the sole cause of what happens.