By IndiaHub 
If you have filed your ITR on or before the due date i.e. September 16, 2025, then the interest calculation period will start from April 1 and continue till the date you get the tax refund. However, if you file your ITR after the deadline, then the interest calculation period starts from the date you filed the ITR till the date the tax refund is given.
For different types of tax refunds, the Income Tax Department pays interest at 0.5% per month or part of a month, but the period for which interest is calculated depends on the nature of the refund:
If you are due a refund on TDS, TCS, advance tax, or taxes paid under Section 199, the Income Tax Department pays interest at 0.5% per month or part of a month. This interest is calculated from April 1 of the assessment year until the date the refund is issued. However, if you filed your return late, the interest period begins from the date you submitted the return and continues until the refund is processed.
If you have paid self-assessment tax under Section 140A, the Income Tax Department provides interest at 0.5% per month. This interest is calculated from the later of either the date you filed your return or the date the tax was paid, and it continues up to the date the refund is issued.
For other types of refunds, interest is paid at 0.5% per month from the date the tax or penalty was paid until the refund is issued.