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We have to convert the Hindu calendar (the lunar one) to the Gregorian calendar. According to the (Dutch) Wikipedia (sorry for that, but it has more information than other websites), it is based on he angle between the sun and moon. Now I have many questions about that, such as what angle do they mean? If I draw a line between the sun and moon, I am missing a line to calculate the angle. But the more concrete question is: how can we convert from the Hindu calendar to the Gregorian calendar? Also, we have noticed that the conversion is not injective; it can happen that one Hindu date relates to two Gregorian dates. We decide in that case to pick the first Gregorian date, but we are still not able to convert dates.

One of the days we know the conversion of is:

Basant Pachami (d-m-Y) : 5-11-2068 (Hindu) : 1-28-2012 (Gregorian)

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Since this is a math site, you should supply sufficient information about the calendars to make it a math question; as it stands you are asking for a general formula given one particular instance of it, which is impossible. For your angle question, it would seem it is the angle between the lines to the sun and to the moon emanating form the (earth based) observer. –  Marc van Leeuwen Jan 21 '12 at 9:33
    
I think this may be of help: en.wikipedia.org/wiki/… –  K. Rmth Jan 13 at 18:37
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I don't have the entire answer but I hope this will at least help a bit you if not more.

Have you looked at http://www.webexhibits.org/calendars/calendar-indian.html

Quoting: "All astronomical calculations are performed with respect to a Central Station at longitude 82°30’ East, latitude 23°11’ North."

Why do you wish to convert these dates? If this is a one-time thing, you might be able to find something that does it for you.

I found http://www.rajan.com/calendar/ but this is Gregorian <> Nepali (I was under the assumption that it is the same, maybe it isn't)

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The earth-moon-sun system is a complicated dynamical system. Starting in ancient times, various approximations have been found for predicting its behavior. It may be possible to use such approximations to produce a relatively short piece of computer code that will work for some amount of time into the future. Another alternative would be to use a lookup table. In either case, I have a hard time believing that it makes sense to reinvent the wheel. There must be canned solutions out there. –  Ben Crowell Jan 21 '12 at 16:57
    
We thought so either. But couldn't find anything! In php there are standards for the Jewish calendar and other calendars, but not for Hindu or other lunar based calendars. We can predict new moon, so we can tell when a new month will start. But we aren't able to tell when we have to skip a day. But thanks for the links, we will definitely take a look at them. –  Kevin Jan 21 '12 at 18:34
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