This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

VAT Default Interest Manual

Calculation of interest: Setting the calculation date

The calculation date is the date the tax assessment is made. It is most important that the correct date is entered in red on the front of the VAT 641/642 input document. 

This is because the computer automatically calculates the interest due on any interest liable tax amount up to the day before the calculation date of that assessment.

If you do not specify a date, the computer will default to the processing date of the VAT641/642 which may result in too much interest being charged. This could happen if there has been undue delay in processing the VAT641/642 where no calculation date has been specified.

You should be aware that any undue delay could lead to complaints or criticism from taxpayers. If the calculation date is too early not enough interest will be charged.

It is only possible to specify the calculation date at the time you input the VAT 641/642. Once it has been input it will not be possible to amend the calculation date. In such cases it may be necessary to adjust the period the tax amount is liable to interest using the VAT 663 procedures, see VDIM6000. Alternatively an amended assessment may be appropriate.