Upvote:2
The problem arises that this 180 days period does not start the moment you enter the Schengen area. These 90 days maximum stay must be valid for any randomly picked 180 days.
Let's make an example:
You stayed in the EU for 40 days, left, returned after 30 for 5 more days then left again for 60 days and returned for the remaining 45 days. That means you've stayed for 90 days during a 180 days period.
The first thing you'd think about is, "oh today is my last day, I have to leave." This is not the case. Tomorrow the first day of your first 40 day visit is outside the 180 days period. Thus you had 89 days in a 179 day period and may stay another day, filling that up again to 90 days stay in 180 days.
You can continue this for 40 days. Then you will reach a day when 180 days ago you left the Schengen area. Now you will have to leave for at least 30 days, because for 30 days you have been outside the Schengen area. Thus any day you now stay longer would make it 91 days during the last 180 days.
Upvote:5
Disregard that site. The US Department of State does not make the Schengen rules, does not enforce them, and has shown here once again that they have a particular talent for expressing them confusingly or incorrectly.
As you can see elsewhere on this site, the rule must be considered with respect to every 180-day period. On any day when you enter the Schengen area, you must look at the previous 179 days and count the number of days of presence in the Schengen area, and find that the number is 89 or lower. Similarly, on any day when you are in the Schengen area, if you look at the previous 179 days and find that you were in the Schengen area on 89 of them, then you must leave the Schengen area before midnight.
In practice, thus means that when you have been in the Schengen area for 90 nonconsecutive days in a 180-day period, the first date on which you may reenter the Schengen area will be less than 90 days after your most recent date of departure.
The maximum length of your stay at each entry depends only on how many of your days of presence were in the latter half of the 180-day period that ends on the date of entry, however. If you have been outside the area for at least 90 days, that number will be zero, and you can stay for up to 90 days. If the number is nonzero, then the maximum length of stay iw reduced accordingly.
The EU has created a calculator that will do the math for you.