Here is a way of thinking, based on the number of days on the outside Schengen. Some corner cases seem to look more natural by counting this way than counting the days in Schengen.
Every time you spend a day (from midnight to midnight) entirely outside the Schengen areayou earn a point.
You also earn one point if you spend a day in a Schengen country for which you have a residence permit or valid long-stay visa (type D) of. This also covers the days you enter or leave such a country, as long as you do not go through other Schengen countries.
Each point expired exactly 179 days after winning this point. In other words, a point lasts 180 days, including the day you win it.
When you have at least 90 unexpired points, you can use the "short visit" rules.
Since you earn either one point per day or none, and the points last 180 days, you will never have more than 180 points.
What is the status of a short visit?
The rules on short visits allow you to enter and stay in the Schengen area if
You are a citizen of a country without a visa (and travel with your passport from this country), or
You have a valid short stay visa (type C) (subject to its total duration and number of entries) from all Schengen country, or
You have a valid residence permit or type D visa and travel (for example for professional or tourist reasons) to one or more Schengen countries other than the one that issued them.
Reach 90 points
If you have not been to the Schengen area in the last 90 days, it is clear that you have 90 fresh points. Everything that happened before these 90 days is forever irrelevant – at least as regards the rule of 90/180 days.
Conversely, if you stay 90 days in a row in the Schengen area and you leave on the last possible day, you only have 89 points left the next day. Since you left the area, you will start to earn new points, but your old your points will expire as fast as you can earn new ones, so you will need 90 days to earn a point. do not just replace the one that expires. Once this happens, the previous paragraph applies.
If you have come and gone several times in the Schengen area, you must be more careful in counting your points. This can be complex, which is why the different automatic calculators are practical. The only thing you get is that you never need to look back 180 days. Everything that happened more than 180 days ago is still irrelevantbecause all the points you earned at that time will have expired.
It is not necessary (and unnecessary) to attempt to keep track of "when a new 180-day period begins", counting since the first day you set foot in the Schengen area there Years ago. (For several months, at the beginning of the 2010s, the law stipulated that it was necessary, after the Court of Justice of the European Communities had ruled that it was so that it was necessary to interpret the law. former ambiguous text of the Border Code: a modification of the regulations to make the current system unambiguous).
Alternatives to short visits
There are also some ways to enter and be legally present in the Schengen area without to be on a "short visit". Here's how you can legally enter if you do not have (yet) 90 points:
If you have a residence permit or type D visa of the Schengen country where you are.
You will earn one point per day, as described in paragraph (2) above.
If you are in transit you have a residence permit or a type D visa to go to the Schengen country and back.
You will not earn points on transit days, but your score will not matter, as transit is not a short visit anyway.
If the country you are in is accept your presence on the basis of old bilateral treaties outside the Schengen framework. This is only available for certain citizenships, only in some countries of the Schengen area. In some countries, having applied because a residence permit will put you in a similar situation until you get a decision.
You will not earn points, so if you want to use this to go beyond the 90-day barrier, you have to put this part of your visit latest.
If you like freedom of movement because you are an EU / EEA citizen or a qualified family member of a citizen.
In this case, the 90/180 rule does not apply to you, so you do not need to count the points. Probably if you to lose freedom of movement rights, you can then act as if you have accumulated points all the time – based on common sense extrapolation of the rules – but that does not say it so often.