The Devout in the US face this problem of finding out the dates of Hindu Festivals,Thithis, Amavasya,Upakarma ,Gayatri Japa and other Festivals like
Deepavai Makara Sankaranthi.
The general practice as I understand from the US and UK is that the priests there use some kind of Calculators, convert the days /dates directly and
arrive at the Information.
I received some information on this,
“ Let us consider the recent lunar eclipse which was observed in USA (15 April 2014) and not in India. It should occur only on a full moon day even in USA as it is in India. According to Srirangam punchangam the pournami was approximately from 2pm Monday(14th April) to2pm Tuesday in Srirangam. This puts the middle of Poornima about 2am on Tuesday in Srirangam. The total lunar eclipse occurred about2am central time, Tuesday, in USA. This clearly shows that we should apply Indian timings as they are to correspond to the same timings with sun rise correction in USA.
I was taken aback when some temples in USA observed Gayathri Japam on the day when lunar eclipse was going on about 4, 5 years back asking followers to wait to do the japam until the eclipse came to an end instead of doing Upakarma on that day ( that day being a pournami as predicted in Indian punchangam ). Gayathri japam should be done early morning after Sandya vandam on the following day of Upakarma. There are no Doshams attached to this.
The USA temple priests are not punchanga nirnayam experts eventhough they are asked to act like one. The Indian astrologers still believe the earth to be flat and at the center of our planetary system. So their opinions to translate the timings to suite other geographical regions are not acceptable.”
What is the correct procedure?
Find the time of Sun Rise.
Check the Panchaangam for Thithi , Nakshatra presence in the day in Naazhkai.
Covert the Naazhikai into normal time.(One Naazhikai-24 minutes)
In conjunction with the Sunrise find out the presence of Thithi on a particular day.
Those who follow Chaandramaana,(Lunar Calendar), must celebrate the function on the day when the Thithis is least(Thithi should be less than 51 Naazhikai)
For more on this please refer my posts on this subject under Hinduism/Time.
The reason to say that direct conversion is incorrect is that our Panchanga Timings are not Absolute.