Generally the tax on income crystallizes only on completion of the previous year. However for ease of collection and regularity of flow of funds to the Government for its various activities, the Income tax Act has laid down payment of taxes in advance during the year of earning itself. Taxes may also be collected on your behalf during the previous year itself through TDS and TCS. If at the time of filing of return you find that you have some balance tax to be paid after taking into account your advance tax, TDS & TCS, the short fall is to be deposited as Self-Assessment Tax.
