Why does the ISKCON calendar show a different Ekadashi date?
Because it applies an additional rule — and both are applied correctly to the same astronomy.
The two rules
Smarta. Observe on the day the Ekadashi tithi is present at sunrise.
Gaudiya (ISKCON). The same, and additionally Dashami — the tenth tithi — must have ended before Arunodaya, roughly 96 minutes before sunrise. If Dashami is still running at Arunodaya, the day is viddha (touched), and the observance moves to the following day.
In our own calculations across 22 locations and ten years, the two reckonings differ on 18.0% of Ekadashis.
Worked examples, Delhi
Prabodhini Ekadashi
Delhi. Ekadashi tithi 2026-11-20 07:16 to 2026-11-21 06:31.
Pausha Putrada Ekadashi
Delhi. Ekadashi tithi 2027-01-18 10:27 to 2027-01-19 07:49.
Nirjala Ekadashi
Delhi. Ekadashi tithi 2027-06-14 02:12 to 2027-06-15 02:08.
Kamika Ekadashi
Delhi. Ekadashi tithi 2027-07-29 07:34 to 2027-07-30 05:13.
Which should I follow?
Follow the tradition your family or temple follows. If you are connected to ISKCON or a Gaudiya Math temple, use the Gaudiya reckoning; most Hindu households in India follow the Smarta reckoning.
Neither is a mistake. They are different rules, applied correctly, to the same astronomy.
A note on terminology
We say "Gaudiya (ISKCON)" rather than "Vaishnava". Our second rule set is validated against the ISKCON/Gaudiya calendar tradition only. We have not checked it against Sri Vaishnava, Madhva, Vallabha, Nimbarka or Swaminarayan practice, each of which may reckon differently. What else we don't know →