Gaurabdha Calendar, GCal

Hare KRshNa~
praNAm
Shrila PrabhupAd ki jay

I have a question about the Gaurabdha Calendar, the Gaudiya VaishNav lunar calendar.

99% of the times, in the Western hemisphere or particularly North America, Ekadashi, or any festival, falls a day before the same event in India.

For instance, i hear it is Balaram Jayanti today , Wed 8/1/2012 in N.A. whereas it is on Thursday 8/2/2012 in Mayapur and rest of India.

Now, granted that it is Thursday in India already when it is only Wednesday in North America. Is this the reason? So that all devotees all over will simultaneously follow the event?

If the tithi is lunar, wouldn't the local longitude matter? It seems some lunar calendars see the tithi differently.

Is it the solar and lunar taken together that makes a difference?

Although it shouldn't matter, it would be nice to know the basic idea of calculating a tithi.

Thank You very much
Hare KRshNa~