Every year, some families notice their relatives in India are celebrating a festival a day before or after them, even though everyone is using the same lunar calendar. This is not an error, it comes from how tithi actually works.
A tithi is not a fixed 24-hour block
A tithi is defined by the angle between the Sun and Moon, and it starts and ends at a specific moment in time, not at midnight. That moment happens at the same instant everywhere on Earth, but local clock time differs by time zone.
Why the date can shift by a day
If a tithi begins at, say, 11:30 PM IST, that is only 1:00 PM the same day in New York (Eastern Time). But if the tithi ends before sunrise in one location and after sunrise in another, the festival can end up assigned to different calendar dates depending on which sunrise it is measured against, since many festival observances are tied to the tithi prevailing at sunrise.
This is normal, not a mistake
Regional panchangs within India itself occasionally show a one-day difference for the same reason, long before time zones abroad enter the picture. Neither date is “wrong,” they reflect the same astronomical event measured against a different local sunrise.
Check the date for your location
Use the panchang page with your own city selected to see which date the tithi falls on where you actually are, rather than relying on a date meant for India.
This information is descriptive.