For example, if the start date is January 1, 2019, and the end date is March 15, 2021, the formula would be =MONTH(end_date)-MONTH(start_date)+12(YEAR(end_date)-YEAR(start_date)) . This will give you the number of months between the two dates.
Even a single extra payment made each year can reduce the amount of interest and shorten the amortization, as long as the payment goes toward the principal and not the interest. Just make sure your lender processes the payment this way.
For example, if you borrow Rs. 10,000 at an annual interest rate of 6% for 3 years (36 months), the monthly EMI would be EMI = 10,000 (0.06/12) (1 + 0.06/12)^36 / ((1 + 0.06/12)^36 - 1) = Rs. 303.87.
You can ask your lender for an amortization schedule, but this might not be as helpful if you're looking to see how extra payments could impact that schedule.