How to track Income Tax Refund?

[Russell]

@Diti_Savalia

If your refund is delayed, the ITD pays you interest under Section 244A at 0.5% per month (6% annually). This is added automatically along with the refund, provided you filed your return within the due date.

For refunds filed within the due date, as you write, what date is used to calculate the period for which one earns interest? Is it the verification date, the process completion date, the due date or some other date?