Tax will be calculated for the whole financial year i.e. from April to March. Also, employees can submit their investment proof at any point of time, however, for our facility, we are accepting them in January / February.
If anyone fails to submit his / her investment proof in January (like you say), tax will be calculated on the earned income from April to December at actual and proposed earning from January to March. The total tax amount will be divided into 3 equal parts and start deduct from January.