90/180 day rule

It is very confusing indeed. I use the Schengen visa calculator on the Schengen visa website, I guess that is official, and then a Schengen planner app which I pay a couple of euros a year for (free initially I think).

 I find the latter easier because it is more visual in that it colours in blocks on a calendar which is easier to understand. They are both clunky, I don’t think there is any other official help out there on it.

It’s amazing as so many people have to use the calculation that there isn’t a simpler app.

Not sure the Schengen calculator is accurate, I travel a lot and track my days using excel in every country I visit. I did a test and what I would consider "a day" is where I am at midnight, so what I would consider a 4 day stay i.e. arrive on Monday leave on Friday the calculator says it's 5 days. Surely it's not possible to be classified as being in 2 countries on the same day???

Yes it's possible to be in two countries on the same day. The Schengen rules don't take into account where you are a midnight, they just look at - did you spend any time in the country on a specific day. The Schengen calculator is correct. You were there on 5 days. M, T, W, T, F.