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.

The family group chat problem

This plays out every year in WhatsApp family groups. An uncle in Mumbai sends “Happy Ganesh Chaturthi!” on Friday, and a cousin in California is confused because their local panchang shows Chaturthi tithi prevailing at sunrise on Saturday. Both are correct for where they are. The confusion gets worse when someone shares a screenshot from an app that defaults to IST regardless of the user’s location.

The simplest way to handle it: celebrate on the date that matches your own local sunrise. If Chaturthi tithi is active at sunrise on Saturday in San Francisco, that is your Ganesh Chaturthi, regardless of when Mumbai started celebrating. You are not “late” or “wrong.” The underlying astronomy is the same everywhere. What differs is which local calendar date that astronomy maps to when you factor in your own sunrise time. Once a family understands this, the annual confusion usually stops. And for what it is worth, this same issue exists within India too. Families in Mumbai and Kolkata occasionally observe a festival on different days because their sunrise times, and thus tithi-at-sunrise, can differ.

Walking through an actual example

Say Ekadashi tithi begins 9:15 PM IST Tuesday and ends 7:40 PM IST Wednesday. In Mumbai, sunrise on Wednesday is around 6:45 AM, well within that window, so Ekadashi is observed Wednesday. Convert to New York time (9.5 hours behind IST that time of year): the tithi runs from 11:45 AM Tuesday to 10:10 AM Wednesday. New York’s Wednesday sunrise, around 6:30 AM, also falls inside that window, so New York observes Ekadashi Wednesday too, purely by coincidence. Shift the tithi’s start and end by even 90 minutes, which happens constantly given the Moon’s variable speed, and New York’s sunrise could fall just outside the window while Mumbai’s stays inside, producing the gap. Each tithi has its own timing, so the mismatch has to be checked festival by festival.

Which festivals this affects most

Festivals tied to a sunrise-prevailing tithi (many vrat days, some purnima and amavasya observances) are most prone to this shift. Festivals tied to a specific muhurat later in the day, like Diwali’s Lakshmi Puja at Pradosh Kaal, are less likely to move by a full day, since that moment has more buffer after sunrise. That’s part of why Diwali dates tend to align globally more often than an Ekadashi fast date.

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.