Indian Calendar/Panchangam is used everywhere where hindu lives. I even see people posting I want pambu panchangam but I live in USA and not available. The question arises can indian panchangam used outside of India?
People usually align the hindu festivals with the English dates based on indian calendar and then they think if an indian calendar says Janmasthami on a certain date say Sept 2nd 2010, then everywhere in the world it will appear on the Sept 2nd. USA is 10:30 to 15:30 hours behind from India so some of them also think it will come next day in USA, i.e. Sept 3rd 2010. They also argue that my family priest in india told me to celebrate on this date and according to him indian calendar will apply to all places in the world. This is completely wrong assumption.
The English date and the day (Sunday, Monday, Tuesday etc..) changes at the midnight whereas the hindu date or tithi, and the hindu days doesn't change at the midnight. Hindu day changes at the sunrise. All days are from one sunrise to another sunrise. Sunday will start at one sunrise and end at next sunrise.
The hindu date (tithi -- depends on the positions of sun and the moon at any given time) changes anytime as celestial bodies keep moving. They can change anytime between two sunrises. 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 (Tithi)?
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 degrees that gives you 30 equal parts. Means we have 30 dates in hindu calendar divided between the bright half, and (Shukla Paksha, Waxing moon) and the darker half (Krishna Paksha, and the waning moon).
Panchangam contains two types of calculations. One based on the local coordinates like longitude and latitude(Sunrise, Sunset, Moonrise, Moonset, etc)., and the other is based on geocentric planetary positions.
Sunrise, Sunset, Moonrise, Moonset times are always calculated for each and every place on the earth. Local Sunrise, Sunsets are used to find daily Rahukalam, Yamagandam, gulikai, durmuhurtham times.
The other set of calculations are calculated from the center of the earth (geocentric). The positions of heavenly bodies are calculated from the center of the earth and hence they are geocentric. The planetary positions are first calculated from the center of the sun (heliocentric) and then using the spherical trigonometry they are converted to the center of the earth (geocentric). The tithi, nakshatra, yoga and karana and daily planetary positions are based on the positions of the Sun and Moon in the heavens and they are part of the astronomical phenomenon. These are astronomical phenomenon it happens instantly at the same moment on the earth and hence thithis, nakshatra will end at the same time / instance all over the world and we have to convert them to the local standard time of the specific country of interest. When eclipse occurs and it's visible all over the world then one converts that time into their timezone. The planets don't wait for sunrise or sunset at the particular place. They just keep moving, like earth is moving around the Sun.
These planetary position of the sun and the moon is computed based on the local time (longitude, latitude) converted to the Universal Time. The tithi end time is also calculated in the Universal time which is then converted to the local time of the place. The planets are calculated from the center of the earth their positions at the given time will be same all over the world. The same way to observe any celestial event occurs we need to convert it to our local time. Indian Panchang makers convert this to Indian time. The very same say we need to convert it to our time zone for America (PST/PDT, CST/CDT, MST/MDT, EST/EDT). After adding the time difference we get the resulting english date and time when they end. 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, and sunrise, sunsets.
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. The same way Western Australia is 8 hours ahead of GMT and hence the chaturthi will end there at 12:09 PM on Sept 12th. The planets don't wait for appropriate tithi to arrive on some english date at any location in the world. If it really did we won't have any days and nights (If it happens then celestial bodies will remain static in sky). The earth is round and it keeps moving, we have days and nights. It doesn't wait for tithi or festivals. Time never waits for anyone. It goes on. The sun doesn't rise instantly at the same time all over the world. Hence we need to convert the tithi, nakshatra, yoga, and karana time to local standard time. Now we know what time a given tithi ends it's a time to decide festival dates for each region.
How festivals are determined?
The usual rule to observe festival is whenever that tithi prevails at the sunrise time. But each festivals may have different rules of observance. 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. Sometime it could be observed on the same day as the date mentioned in India. America is behind india in terms of time. To arrive at the time from india we need to subtract the time from indian time and hence festivals could arrive a day early in usa but not later. For example if a total lunar eclipse is visible in india at 5:00 AM we’ll have a previous day in USA, not the next.
For these reasons Indian panchangam you buy in India cannot be used outside of India. Even sometimes festival observance dates could vary between Kolkata to Mumbai. So Forget about using it outside India. If you are looking for precise panchangam for your city please visit mypanchang.com