As a green card holder, one can't stay outside the country for more than 180 days. I'm trying to learn to count the days correctly.

>= 180 Days: A risk of being considered as an abandoner. 
>= 360 Days: Considered abandoner. 

Is my arithmetic below correct? For example, I boarded the flight on Dec 31, 2021, to exit the country.

The count starts from Jan 1, 2022. Is this a correct assumption?

Jan 2022 - 31 Days
Feb 2022 - 28 Days 
Mar 2022 - 31 Days
Apr 2022 - 30 Days
May 2022 - 31 Days
Jun 2022 - 30 Days (Total 181 Days by end of June)

Ok -> I should land in the US on Jun 28 and satisfy the condition < 180 days.

Not Ok -> If I land on Jun 29 or later.

P.S. In real life, I'll attempt to be on the safe side by returning much earlier, just trying to get my Math right here using this edge case.