Quarterly Instalment

Giant Ltd. is a large company, and therefore pays its corporation tax by instalments.

For the eight-month period ended 31 August 2025, the company estimated that its corporation tax liability would be £400,000.

What would Giant Ltd’s first corporation tax instalments be in respect of the eight-month period ended 31 August 2025, and what is the due date of this instalment?

A. £100,000 on 14 April 2025
B. £100,000 on 14 July 2025
C. £150,000 on 14 April 2025
D. £150,000 on 14 July 2025

My answer was £150,000 on 14 April 2025.

The answer at the back is D.

I understood that it should’ve been counted from 1st of January. So the date is 14th of July 2025.

But the next thing that is written at the back of the answer is:

For an eight-month period the instalments are due on the 14th day of months 7, 10 and 12 (the last being 4 months after the end of the AP)

I’m confused about the 12th month.
In OT notes it was 13th month.

Is it because the AP ends on 31 January 2025?

I kind of get confused between end of the tax year and accounting period.
I mean getting confused if we should do this according to the tax period or accounting period.

I need your guidance on which ones should I account for according to the tax year and which ones according to the accounting period, if possible. I’m mixing things up a bit.

Thank you.