Leap years and the 365.25-day year in dasha
The dasha years in the Vimshottari table are fixed numbers of years, and the calculation converts them to days using a year of 365.25 days, the Julian average.
What it is
A year of 365.25 days is the Julian year, which averages the leap-year rule. The tropical year is about 365.2422 days, so the Julian year is slightly longer, a difference of about 11 minutes a year.
How it is worked out or read
The dasha tool uses 365.25 days so that the length of each period in days is a simple multiple of the table's years. The difference over a 120-year cycle is a fraction of a day per century, which does not change a period's boundaries in practice.
Where it appears on this site
The dasha lengths are in the dasha tool. The graha dasha years article lists each graha's years.
Why the year length matters
The dasha tables give lengths in years, but the calculation must turn years into days, and that step depends on the length chosen for a year. The tool uses 365.25 days, the Julian average, which spreads the leap days evenly across the centuries. A different year length would shift the boundaries of later periods by a small amount, which accumulates over a long timeline.
The effect is small for a single period and can grow over the whole cycle. A reader who compares the tool with another calculator should check which year length is used, since this can explain small differences in later dates.
Leap years in practice
Leap years are inserted in the civil calendar to keep it close to the tropical year. Over a period of twenty years, about five leap days fall in the interval, and these are included on average by the 365.25-day year. The result is that a twenty-year period has the same length in years whatever the calendar, and its end date falls within a day or two of the calendar anniversary.
A reader who needs exact calendar dates across a leap day should be aware that the civil date may differ from the day count by one, and should use the date shown by the tool for the dasha boundary.
The tropical and Julian years
The tropical year is about 365.2422 days, and the Julian year of 365.25 days is slightly longer. The difference is about eleven minutes a year, which accumulates to about one day in a century. For dasha purposes this difference is negligible over a single period, and the site uses the Julian average for simplicity and consistency with the rest of its calculations.
When the difference matters
For most readers the difference between 365 and 365.25 days is irrelevant, because it changes a dasha boundary by a few days over a whole period. It matters when a reader is trying to match a timeline exactly to a dated event, or when comparing two calculators that use different conventions. In those cases the reader should state the convention and accept that small differences in dates come from it.
It also matters for long-term planning, where a shift of a few days over many periods can add up. A reader planning around a distant boundary should use the tool's dates and not a rough estimate.