accounting period for companies

suppose a comp prepares accounts for 15 month ended period 30 sept 2024

here how do we determine from where to where is 12 month a.p . what i mean is
case 1 if we take 12 months from sept 24 then the first a.p would be 1 sept 23- 30sept24 and the second a.p would be the prior 3 months being 1 june 23 – 30 aug 23.

well this example i gave is part of a qtn of kaplan exam kit
so what theyve done is they took 12 months period as 1 july 23- 30 june 24 being first a.p
and second a.p being 3 mnths 1 july 24- 30 sept 24.

im jut confused how do we know which months will fall into 12 month period nd which will not