In modern Gregorian calendar there is a leap year after a regular interval of four years. After 100 years this leap year doesn’t come and after 400 years it comes again. In this way a year is of 365.2425 days which is very close to 365.2422 days of Sayan year and only after 3000 years there is difference of one day.
In contrast to calendar, Panchang contains the entry time of planets like Sun, Moon etc in different signs along with the calculation of Tithi, Yoga & Karan. When we see planets in context with Stars stars it is known as Nirayan position of planets. The value of a Nirayan year is 365.2563, which is the duration of the re-entry of Sun in one sign. This is bigger than Sayan year by 0.0142 days. In that way there comes a difference of 1.42 days in 100 years. In each 100 years Panchang moves ahead from calendar by one and half day & hence there comes difference in Makar sankranti and Lohri. As a result of that the Hindu festivals which are celebrated according to Tithi move ahead.
What is the difference between Sayan & Nirayan systems? The earth moves on its axis around Sun in an orbit. But there is a tilt of 23.4° in the earth. This tilt on earth is responsible for summer and winter. Because of the tilt that part of the earth, which comes straight in front of Sun, turns hot and results into summer. Seasons & Ayans are because of the tilt of Sun & not because of the revolution of earth. Therefore to maintain same dates for seasons, the calendar is based on Sayan calculation. The middle line (equator) of the earth disects the orbit on one line, the one point of which is spring solstice and the other is winter solstice. This line moves towards west with a speed of 50".3 per year. The complete 360° movement of earth with respect to stars is known as Nirayan year and its coming back to the same tilt again is known as Sayan year. Because of this the earth has to move 50" less from 360° while returning back to winter solstice. This difference is known as Ayanamsa.
Sayan calendar & Panchang were same on 23rd March 285. From that date till today there has come a difference of 24 days. The month of Chaitra, which used to come between February & March, now comes between March and April. The month of Jyeshtha that used to fall in May now falls in June. The summer season, which used to come in Jyeshtha has now shifted to Vaishakh. Similarly the rainy season has shifted to Ashadha - Shravana from Shravana - Bhadrapada. Seasons are based on Sun therefore they prove true according to Sayan calendar only. Calculation of time of change of seasons is not accurate on the basis of Nirayan Panchang based on Rashi (sign). Any calculation based on Sun fits true on Gregorian calendar but shifts in Panchang. According to calendar 23 December is the smallest day and on the same day Sun goes into Uttarayana. But according to Panchaang Tithi or Pravishtey Sun is not in Capricorn sign. Long time back in the year 285 Lohri or Makar Sankranti used to be smallest day but not now.
Should we convert the calendar into Nirayan or else convert Panchang into Sayan? The answer is -No. Both are correct on their own places. The calendar is meant for general public, so should be based on Sayan calculation only so that the seasons and Sun rise etc are according to date. Astrologers for the calculation of the position of planets use the Panchang. The position of planets is according to Rashi therefore Nirayan calculation is accurate. It cannot be Sayan. Therefore all Panchangs are Nirayan only. For the calculation of planetary degrees Ayanamsha should be subtracted from Sayan calculations to convert it into Nirayan. Generally we get the Sayan tables of planetary degrees. But it does not mean that planets are moving in signs according to Sayan calculation. All the planets move according to Nirayan calculations only and astrology based on Rashi and Nakshatra is completely Nirayan. Therefore both the systems of calculations are correct and we should not create illusion by making changes in these calculations or by considering them wrong. Difference of calculation in calendar and Panchang is also true.
There is need for the unification of all the panchangs. Some Panchangs follow traditional Surya Siddhant. In the modern era of scientific research where man has succeeded to reach planets through satellites, it would be merely a conservative approach to advocate the inaccuracy of modern methods of calculations. If the predictive aspect of astrology doesn’t appear to fit the modern calculations, then we need to change the fundamentals of predictive astrology instead of the mathematical aspect. The prediction can not be the basis of mathematics rather mathematics only should form the basis of predictions. All panchangs should follow the international standard of calculation of planetary degrees.
All panchangs should follow the internationally accepted accurate planetary degrees and calculation should be done through computer to assure accuracy so that there doesn’t remain difference in Panchang. It is necessary to do so to overcome the confusion of common man.
The difference of opinion in Aynamsha is also responsible for different planetary degrees. When we fail to recognize the difference between Sayan and Nirayan and do not succeed in telling the native his accurate Rashi or lagna with confidence then what is the use to keep different Ayanamshas. Some Aynanshas are being followed because of their names only. The difference between two Ayanamshas is almost negligible and it is difficult to prove their accuracy. We should overlook the difference of opinion and follow Chitrapakshi (Lahiri) Aynamsa only.
The other difficulty in making Panchang is lack of unanimous opinion about festivals.
In the determination of Tithi the role of sunrise is important. Timing of sunrise is different in different places and that is why Tithi also varies with place. As a result of it there is difference in Panchangs of different places. Eventually the dates of festivals based on Tithi also change. Therefore to bring unanimity it is important to fix the place of festival on the basis of its history.
Festival on all places should be celebrated according to the Tithi determined on the concerned place though in scriptures we do not find the name of place with the festival.
Sometimes one festival falls on two dates. It generally happens with the festivals like Deepawali and Janmashtami. The reason is mid night calculation of Tithi and Nakshatra. Generally Tithi at the time of sunrise is taken on the Tithi for the day. If the calculation of timing of festival is done through proper method, this difference won’t come.
In Hindu scriptures the detailed method of calculation of festival is given and there are little chances of going wrong in fixing the festival.