The Irani community is distinguished from the Parsis, the descendants of Zoroastrians who emigrated to India in the early medieval period; some Iranian Zoroastrians later migrated to India as well and formed a distinct Irani community there. Differences of calendar and custom developed between the two branches over the centuries of separation. The affirmation expresses the Irani sense of continuity with the faith's Iranian roots, complementing rather than opposing the Parsi identity, the two together carrying forward one of the world's oldest living religions. The two communities, and factions within each, additionally differ over which of three traditional calendars to use for religious observance: the old Shahenshahi calendar, historically dominant among Parsis; the Qadimi calendar, adopted after an eighteenth-century recalculation; and the Fasli (seasonal) calendar, a twentieth-century reform, adopted in Iran in 1906 and by some reformist Parsis, that resynchronises the ritual year with the solar seasons.
Because major festivals fall on different dates depending on which calendar a community follows, Zoroastrians in different congregations, even different branches of the same family, can find themselves celebrating the same religious new year, Nowruz among them, on three different days, a persistent source of practical friction within the tradition.