I’m confused

Could someone be nice enough to help me with the correct calculation.
As part of naturalization requirement, the applicant must prove that he/she was in the Uk three years prior to the application date .
I’m due to apply on 1st April 2020 .
Do I need to be in Uk on April 2017 or 2018 ?
This is how I calculated it .
1st April 2017 to 2018 -1yr
1st April 2018 to 2019 - 2years
1st April 2019 to 2020 -3 years
It’s very important I know what date to use because I was out of the country from March 2018 to 2nd April 2018.
I was told my three years is April 2017 not 2018 but based on my calculation, I believe it to be 2018 .
Please help me get out of this confusion . What date exactly is my three years prior . Please don’t forget is for 2020 application .
Thanks