Salary for the month of March is credited in the month of April

It is based on you Form 16 

There is nothing wrong in disclosing the income in 2020-21 as accused income not paid during a financial year is a bonafide transaction of that Assessment year.

This has happened to all the Employees of Central Government and hence there is no exception. 

Since the March Salary was paid in April, your Form 16 issued to you by your Employer shall have 11 salaries only paid during the previous financial year. Please verify that first. If it has eleven salaries only you have the following options:

1. File your IT Return as per your Form 16 in which there shall be 11 salaries. You need not make any explanatory notes or rejoinder etc to The I T Department.
2. File your IT Return as per the salary earned during the financial year ie. include the 12th salary for March in your IT Return for the financial year although it is paid and received by you during the next financial year. If you do this, there shall be a mismatch between your Form 16 submitted by your Employer and the IT Return submitted by you in which case:

           a) make a note in your present ITR as to why you included salary paid in April, in this ITR

           b) In the next year Form 16 salary paid in April shall be included in the present financial year and therefore, you need to include it in the IT Return of that                   financial year. Therefore, you need to add an explanatory note stating that it was already included in the previous financial year.

Discuss the matter with your colleagues and do as everybody is doing. Maybe your Union shall advise you properly in the matter giving a common approach by all Employees.

Income and expenditure for the period 01.94.2020 to 31.03.2021 shall be calculated.