How we calculate Ekadashi dates
Where do the dates come from?
We calculate them.
Ekadashi is the eleventh tithi — a tithi being each 12 degrees of separation between the Moon and the Sun. We compute the Sun's and Moon's positions from published astronomical theory, find the moment that separation reaches 132° (Shukla) or 312° (Krishna), and find your local sunrise.
We do not copy dates from another calendar. We compute them, and then check our results against published calendars to see whether we agree.
We use two independent astronomical libraries and compare them against each other. They agree on tithi boundaries to within about 15 seconds.
How is location handled?
Ekadashi depends on when the sun rises where you are.
We use your city's latitude, longitude and time zone. Sunrise in Los Angeles is about 13 hours after sunrise in Delhi, so the tithi in progress at your sunrise is often not the one in progress at theirs.
This is why your family in India may observe on a different day. In our own data, Delhi and Los Angeles fall on different days for 56% of Ekadashis, Delhi and New York for 41%.
Both are correct. Ekadashi has always been local.
Daylight saving is applied automatically using the standard time zone database.
Why can two calendars differ?
Three reasons, all legitimate.
1. Location. As above.
2. Tradition. Smarta observance follows the day Ekadashi is present at sunrise. Gaudiya (ISKCON) observance also requires Dashami to have ended before Arunodaya, about 96 minutes before sunrise. In our data the two differ on about 18% of Ekadashis.
3. Rare tithi shapes. Sometimes a tithi covers two sunrises, or none at all. Traditions resolve these differently.
A difference between calendars is usually not an error in either one.
Calculation versus religious rule
There are two layers, and we keep them apart.
Astronomy. Where the Sun and Moon are. Not a matter of opinion. Two independent libraries, agreeing to within 15 seconds.
Religious rule. Which day that makes your fasting day. A matter of tradition, and traditions legitimately differ.
We are confident about the first layer. The second we can only describe, tradition by tradition — which is why we show you the rule we applied rather than only the answer.
What happens when the calculation is uncertain?
We tell you.
Sometimes a rule turns on a margin of seconds — whether Dashami ended just before or just after Arunodaya. When that margin is smaller than the disagreement between published almanacs, we say so, and we show you the margin.
We would rather show you a two-minute margin than a false certainty.