Hindu calendar and Panchangam are Lunisolar based calendar and need positions of the Sun and the Moon to calculate Hindu dates and various combinations of the Sun and the Moon in the day e.g. Tithi, Nakshatra, Yoga, Karana and weekday. The day or some part of it might be considered auspicious or inauspicious depending on these combinations.
Panchangam and almanac makers, also known as Panchangam Karta, in India use two different methods to get stars's positions. In Vedic Astrology both the Moon and the Sun are considered as stars.
One group of Panchangam makers uses old methods which were quite handy to give correct positions of stars without doing lengthy calculations. Old calculations were written in form of statements or Vakya for easy memorization and understanding. Those shortcuts used to get updated time to time to remove any inaccuracies which creeps in such methods with time. The calculated positions of stars by Vakyam and Surya Siddhantic methods were exact enough to match with observatory positions of the stars without much effort.
These old methods have not been updated since centuries and star positions calculated by these methods are erroneous. The difference between calculated and observatory positions might vary up to 12 hours. Hence the positions of star calculated by old algorithms like Vakya Siddhanta or Surya Siddhanta are not precise. Followers of these old schools are well aware of these discrepancies but still use erroneous methods. However they don't mind stealing eclipse information from Drik Panchangam or Thiru-Ganita Panchangam.
The other group of Panchangam makers, like DrikPanchang.com, uses modern algorithms to get position of stars or use NASA ephemeris. In computer era number crunching calculations are not a problem. Drik Ganita Panchangam makers use modern algorithms to get precise position of stars and use those positions in Panchangam. The government of India has also supported modern ephemeris for National Panchang or Rashtriya Panchang and Positional Astronomy Centre in Calcutta publishes modern Indian Ephemeris since 1957.
In other words, difference between Drik Ganita and Surya Siddhanta is not very deep. Both schools use exactly same rules to calculate Hindu dates and festivals. The difference is due to the used method to do arithmetic calculations to position various stars and planets in the sky.
In Tamil Nadu many local calendars follow Vakyam Panchangam and to match Tamil dates with local calendars we also support Vakya Panchangam including Thiru Ganita Panchangam.
By default our Tamil Panchangam use modern algorithms but one can switch to old Surya Siddhantic algorithms by clicking Switch to Vakyam button on Tamil Panchangam page.
For Vakya Panchangam all links to other pages are disabled as most other pages conform to Thiru Ganita. By switching to Thiru Ganitha one can access links to other pages on the site.
Thiru Ganita Panchangam is also known as Tiru Kantia Panchangam.