Why do Hindu festival dates differ by city and country?

A Hindu festival falls on a specific tithi (lunar day), and the tithi that counts is the one running at sunrise (the udaya tithi). Because sunrise happens at a different moment everywhere on Earth, two places can be on different tithis on the same calendar day — so the festival can land on a different date in, say, New York versus Mumbai. This is why your local panchang sometimes disagrees with one printed in India.

The tithi doesn’t line up with the clock

A tithi is the time the Moon takes to gain 12° on the Sun. It can start or end at any hour and lasts anywhere from about 19 to 26 hours — it isn’t tied to midnight. So whether a given tithi is present at sunrise depends on your longitude and the exact sunrise time at your location.

A quick example

Suppose the Amavasya tithi for Diwali ends at 3:00 PM IST. In India, Amavasya is present at sunrise, so Diwali is that day. In the USA — many hours behind — the same instant is the previous night, and by local sunrise the tithi may already differ, shifting the observance by a day. Festivals anchored to special moments (Janmashtami at midnight/Nishita, Karva Chauth at moonrise) can shift for the same reason.

So which date is “correct”?

Both are correct — for their own location. The right approach is to follow the panchang computed for yourcity, which is exactly what these pages do: every date here is derived from your city’s precise sunrise using the Lahiri (Chitrapaksha) ayanamsa, and validated against standard Vedic almanacs.