The period of one year is usually preferred across the globe for the balance sheet and income statement of companies across the globe. This one-year period starts on 1st April and ends on 31st March in India. This period is called Financial Year or Fiscal Year.
Tax returns are filed and taxes are paid to the government by corporations in the year following the end of the Financial Year. This year in which the tax is assessed is known as the Assessment Year.
The accounts for the year starting 1st April 2019 to 31st March 2020, this period is called Financial Year 2019-20. The income would be assessed to tax in the next year and that will be called the assessment year 2020-21. The due date for filing income tax returns is 31st July/30th September of each Assessment Year.