"Interest shall be calculated at the rate of one-half per cent for every month or part of a month comprised in the period from the 1st day of April of the assessment year to the date on which the refund is granted"
So Interest on refund is given from 01st April till the date of passing of order.
In case of belated return section 244A(2) will come into play:
"If the proceedings resulting in the refund are delayed for reasons attributable to the assessee, whether wholly or in part, the period of the delay so attributable to him shall be excluded from the period for which interest is payable"
So period of delay (In months) by assessee will be reduced from the period for which interest is required to be paid.