Do layovers count in the 90-in-180 Schengen rule?

Question

Suppose someone on the 90 of 180 rule leaves Schengen on the 90th day.

Then less than 90 days later, they have a flight with one or more layovers in a Schengen country.

Does it get counted if it's the same plane? Change of planes?

What if it is overnight but you don't leave the airport?

How to calculate stays against 90/180 visa rules? relates but doesn't specifically mention layovers. If you don't get off the plane, I presume nobody even looks at your passport. But in one non-Schengen airport, I had to change planes, but first was required to go through passport control, claim luggage, and re-check the luggage.

Answer 1

It's actually very simple, there are no complex rules on how to keep time, what counts or not, or anything like that. Each time you cross an external border, you should get a stamp with a date on it. The day you got an entry stamp, the day you got an exit stamp and every day between them each count as one day towards the 90-day limit.

So if you enter and leave immediately, or during the same calendar day, it counts as one day, whether it's a day trip, en errand to run, a layover or what not. In fact, if you enter 1 hour before midnight and leave 2 hours later, then your passport would show two days of stay in the Schengen area.

On the other hand, if you can transit without leaving the transit lounge/“international” area of the airport (and major European airports are set up to allow that in many cases), it doesn't matter. You won't (need to) go through an official border check or be admitted to the Schengen area and won't get a stamp so there is no need to count days. But if your route includes a flight between two Schengen airports or you somehow need to go “landside”, you have to go through the border check and get a stamp.