Depends on the details of how you define calculus. Archimedes was doing integration, Descartes was evaluating slopes and tangents of algebraic curves. Isaac Barrow had a good grasp of fundamental theorem of calculus -- that differentiation and integration are inverse operations. Brook Taylor did Taylor series before Newton.

Newton and Leibnitz get credit because they placed calculus as a general technique that is immensely broadly applicable not just for extrapolating the tangent function or the sin function as a series, but to any function that's smooth in some sense.

They worked out the details that do not depend on the specifics of the function and called them out as rules/results of broad applicability. Especially how to push the differential and integral operations through +,-,×,÷ and function composition sign. It did not matter what the function was as long as it was built up from those operations.

I am familiar with the work of Kerala school and also of Aryabhatta's work on using differential coefficients to extrapolate the sin function (this being much before Kerala school), his work on difference equations.

Rather than getting caught up with us versus them narratives , spend some time learning about the beauty of math and how different cultures have thought about them in such creative ways. Otherwise you risk sounding ignorant and rageful conspiracy monger.

The inaccuracies in the Hindu calendar is and was a lot more than the Julian calendar. The Julian calendar assumed that a year was 365 1/4 the day. This is slightly inaccurate and therefore there was a need to correct for the leap day that's added every 4 years by skipping it every 100 years, adding it back every 400 years. At time of Gregorian correction it was off by 10~11 days because of an error of 10 minutes in their estimate of the length of an year.

Hindu civilization's estimate of the length of an year, although remarkably accurate for its time, was less accurate than the estimate of 365 1/4th. Usually the length of an year has been overestimated, making the Hindu calendar lose accuracy quicker. These errors were order of a day per year versus off by about 10 minutes like the Julian calendar.

This was less of a problem historically because Ujjain observatory would correct the calendar time time using accurate observation of the Equinox. Since the fall of Ujjain observatory the Hindu calendar has been accumulating drift error for centuries.