Before filing of return within the due date u/s 139(1), the assessee is required to pay self assessment tax u/s140A......When the return is processed under section 143(1), any tax becoming due from the assessee shall be sent an intimation under that section and such intimation shall be deemed to be notice of demand u/s156........if the assessee doesn't comply by the demand notice within 30 days, the assessee shall be deemed to be in default from the date immediately after 30th day..
As Sec.220(2) provides......
"the assessee shall be liable to pay simple interest at [one] per cent for every month or part of a month comprised in the period commencing from the day immediately following the end of the period mentioned in sub-section (1) [ i.e. 30 days.] and ending with the day on which the amount is paid :"