Missing invoice amount

If the invoices were not mentioned in the GST return for the particular month, then report the same in next month GST return.

Whether the query was related to GST?

Can you make it detailed?

If it is related to missed invoices in filing GST return add those in the next month and file.

If it is in GSTR 3B interest for the tax amount to be calculated from the due date of the month to which that invoice belongs till filing date in which that missed invoice added and tax paid. 

Interest at the rate of 18%. 

Example:

Taxable sales of Rs.55,555 missed in a month Dec 2019 while filing GSTR 3B. The missed invoice if filed with addition to Jan 2020 return filed on 19/02/2020.

The due date for Dec 2019 is 20/01/2020, invoice filed on 19/02/2020 with Jan month return difference between two dates in 30 days.

The calculation would be

Tax Amount 55,555 * 18% = 10,000 

Interest = Tax amount * 18% * No of days delay / 365 = 10,000 * 18% * 30 / 365 = 148.

Interest is calcuted on net tax laibility or gross tax liability i.e interest will calculate on after adjusting ITC or before adjusting ITC.