within /after the end of accounting period

Hello

1) Sir, if suppose company’s year end is 30 April 2019, and if we are asked that when it will file its corporation tax return. So we know that it will file it 12 months after the end of accounting period, i.e by 30 April 2020.

2) In IHT, if donor has made a PET on 1 oct 2015. Now if he dies within 7 years of gift i.e by 30 sept 2022, death tax will be payable

My question is that in first case 1 year will be completed from 30 April 2019 to 30 April 2020, however, in second case 7 years will be completed from 1 oct 2015 to 30 sept 2022 rather than 1 oct 2022. So why is is this difference. I hope you have understood what I am trying to ask