Why there is a one day difference in hindu festival dates in India and America?

Also found at: http://www.mypanchang.com/Why-there-is-a-one-day-difference-in-hindu-festival-dates-in-India-and-America.php

The above statement is usually made by many devotees of the Hindu temple of Seattle (http://www.htccwa.org). I am sure this question must have arisen in everyone's mind. The above statement is not entirely accurate. We buy indian calendar and because we use english date for our day to day work we try to align the festivals with the English date. There is a basic difference between these two dates. The English date and the day (Sunday, Monday, Tuesday etc..) changes at the midnight where as the hindu date or tithi (the festival falls in to) doesn't change at the midnight, and hindu day also doesn't change at the midnight. Hindu day changes at the sunrise. So we have a day from one sunrise to another sunrise. That's called the hindu day (Sunday, Monday etc...). The hindu date (tithi -- depends on the positions of sun and the moon at any given time) changes at anytime as celestial bodies keep moving. They can change anytime between one sunrise to another sunrise, between one midnight to another midnight. This means if prathama tithi ends today at 23:02 it doesn't mean that dwitiya tithi will end tomorrow at the same time. It can end at 18:00, 18:09 or 28:27 (anytime after 24 means time after midnight but before next day sunrise), or whatever.

Now what's the hindu date?
It's simple tithi is a lunar day, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours. So maximum distance between the sun and moon can be 360 degrees. If you divide them by 12 that gives you 30. Means we have 30 dates in hindu calendar. Now there is a bright half (Shukla Paksha, Waxing moon) and the darker half (Krishna Paksha, and the waning moon). Now because this is based on the actual movement of the sun (earth + 180 degrees) and the moon they don't necessarily change at the midnight or the sunrise. The planets don't wait for sunrise or sunset at the particular place. They just keep moving, like earth is moving around the Sun. Hence the distance we call it tithi (quotient or the result of the division = (sun - moon)/12.0) can change at any given time. The reminder is called the remaining time it will take to enter new thithi. So new thithi can change at 6:00 AM, 6:32 AM, or 11:57 PM or any time during the day. There is no fix time like english date. Now these planetary position of the sun and the moon is computed based on the local time (longitude, latitude) and then converted to GMT time and then Universal Time. The tithi end time is also calculated in the Universal time which is then converted to the local time of the place. That's because planets are calculated from the center of the earth -- Geocentric. Hence, their position at the given time will be same all over the world (After subtracting the time difference). The same way to observe any celestial event occurs we need to convert it to our local time, and also the same way this thithi, nakshatra, yoga and karna positions are same at any given time in the world. So panchang makers convert this to Indian time. The very same say we need to covert it to our time zone for America (PST/PDT, CST/CDT, MST/MDT, EST/EDT). After adding the time difference they'll assign to the resulting english date.Hence, when the calendar is made of india the date they mention is when that tithi will end as per the english date in india. If you take the same date and time use it anywhere in the world as it is then we have a wrong result because of the time difference. The time difference between the india and the London is 5 hours 30 minutes, Time difference between Seattle, WA and India is 13 hours 30 minutes. The planets don't remain static during that time. They keep on moving and hence at the same time and date in different parts of the world we will have a new tithi. But if you add and subtract the time difference from the indian calendar you'll arrive at the time when that tithi will change in your locality too. Too complex? Let me give you one example.

For example:
October 17th 2008 the Ashwayuja (Ashwin) krishna Paksha / Poornimant Kartik Krishna paksha tithi Tritiya ends at 16:39:21 Indian Standard Time, and chturthi starts. That means at 16:00 hours there is tritiya still prevailing in India. Now on October 17th 2008 in Seattle, WA at the 16:00 hours we won't have tritiya at that time. We already have chaturthi started why? a) Planets keep on moving.... 13 hours have passed and the position of sun and moon have changed. So what time Tritiya will end in seattle? The time difference is 13 hours 30 minutes (with one our day light saving time it gives the difference of 12 hours 30 minutes from IST). So, let's subtract 12 hours 30 minutes from the 16:39:21. That gives us 04:09:21 AM on October 17th. So on October 17th after that time we'll have chaturthi tithi and before we have tritiya. The same way if tithi ends at 8:39 AM on Sept 12 in India it will end at 20:09 PM on Sept 11th in Seattle.

How festivals are determined?
The usual rule to observe festival is whenever that tithi prevails at the sunrise time. But for certain festivals rules change, for example: Ganesh Chaturthi. Sankathara (Sankshathi chauth) chaturthi, Janmashtami, Mahashivaratri, Karwa Chauth, etc... For example Ganesha chaturthi has to be oserved when the chaturthi tithi is observed during the 8th/15 part of the dinmana or the 3/5th part of the dinmana. The dinmana is the difference of local sunset and the sunrise on the same day. If chaturthi is not prevailing during that period then take the second day. Likewise for the karwa chauth and the sankathara chaturthi the chaturthi tithi must be prevailing during the moonrise time, if it doesn't prevail then take the day where it prevails during the sunrise. So for example if one thithi is observed at the moonrise time in india it may or may not be observed during the moonrise time on the same day in the different part of the world. It may be very well observed on the previous day if you are in America or the next day if you are in Japan, Fiji, Australia and other countries, depending on the local events like sunrise, moonrise, and time difference. So sometime it may very well could be observed on the same day as the date mentioned in india.

Now if panchang makers in india tries to keep each and every place in mind then they will be creating panchanga for every city, town in the world. Who'll buy it? Printing costs etc... So they just make panchang for the one of their city in india. Now given that example even the IST remains same for kashmir to kanyakumari and from kutch to west bengal the festival dates could be very well different in certain city. For example for sankathara chaturthi moon rise time is important, same thing for karwa chauth. Now let's take an example: Moonrise in new delhi on a given date is at 19:36 and in west bengal, Kolkatta is at 19:25 PM. Now tritiya ends at 19:32 PM IST. So Kolkatta will celebrate karwa chauth/sankathara chaturthi on the next day where as people in New Delhi will celebrate on the same day.

Pandit Mahesh Shastri & Dr. Ramachandra Joisa
Drik Panchang Karta and Panchang Siddhanti
http://www.mypanchang.com/