What is the correct date for leaving Thailand on a Non-OA multiple entry visa?

Query on Non I’m OA multi visa.

After reading a post on here I would like some clarification please on dates.

Our visa is stamped from 14/2/2019 - valid till 13/2/2020

When we arrived in Thailand our passport is stamped

Admitted 8/4/2019 until 6/4/2020

We were planning on leaving the country for short trip on 4/4/2020 but now after reading the post on her we think we

are reading it incorrectly. That we should be leaving before

14 Feb 2020.

Can someone please confirm what date we should be using as I would hate to loose the extra year on the visa. I can then put the date in my phone to remind us.

Thanks

Yvonne