Surya Siddhanta and Drik Siddhanta is a topic of confusion in the world of Panjika.But first we need to know what they really are. They are methods by which we can calculate The Lunar days , Tithi timings and related things which is important for Indian Style Calendars.
It is said that Surya Siddhanta is the first ever book on Astronomy. It is believed that this book is nearly 1500 years old.Surya Siddhanta has rules by which it is possible to determine length of year, position of planets in the sky, solar eclipse etc. It also defines the Earth's diameter and circumference. Eclipses and color of the eclipsed portion of the moon are mentioned as well. The Surya Siddhanta contains the roots of modern trigonometry. Its trigonometric functions are the direct source of the terms sine and cosine. It also contains the earliest use of the tangent and secant when discussing the shadow cast by a gnomon. Surya Siddhanta is important for calendrical calculations. A large group of Conservative Calendar and Panchang makers use Surya Siddhanta to determine date and festival timings.
[Source : Wiki]
On the other hand , Drik Siddhanta uses precise position of planets and uses the same to determine the festival timings and other Panchanga details.So in the age of computers Drik Siddhanta methods gives you the correct and precise timings of important events.
Now lets get into the confusing part. Surya Siddhanta has not been updated with true position of planets and the people following it are still using the old values which produces erroneous results in terms of calculation. Now for Indian Calendar, the position of Sun on a Rashi is used to determine the length of a month and when the next month is starting(With few differences in different states ) . Because of the difference in the calculation almost every year we observe difference in dates . This confusion is mostly seen in West Bengal , because some of the news papers they follow the old one where others follow the new one. So, for those months where the date differs between these two methods we get lots of negative reviews in our app. Our App Bengali Calendar (India) and Assamese Calendar supports both of the methods. It is upto the users which method they want to follow. But for true timings of events Drik Siddhanta is recomended.