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HMRC internal manual

Compliance Handbook

Interest: Establishing the number of days of interest

You must check whether, and from which date, the FA 2009 interest rules apply to the tax or duty you are dealing with. See CH140160 for full details.

We calculate interest from the interest start date to the interest end date. There is more detailed guidance at

Take care to work out the number of days from the interest start date to the interest end date correctly. Follow these examples:

Interest start date Interest end date Number of days for which interest due
31 January 2011 31 January 2011 None
31 January 2011 1 February 2011 1
4 February 2011* 7 February 2011 3
31 January 2011 28 February 2011 28
31 January 2011 1 March 2011 29
31 January 2012 (leap year) 1 March 2012  30
31 January 2011 3 June 2011 123
  • The interest start date in this example arose where we issued an ITSA return after 31 October following the year of assessment, and the due date is 3 months from the date we give the notice under section 8 of TMA 1970.

 

FA09/S101 (3)