The Schengen 90/180 rule is not based on a fixed calendar half-year. The 180-day period moves every day. For every day you are in Schengen, you look backwards and count how many stay days fall within the previous 180 days.
Simple mental model
Imagine a transparent 180-day ruler sliding across your calendar. Each day, the ruler moves forward by one day and older stay days gradually fall out of the window.
Why travellers get surprised
Because the window moves continuously, the number of remaining days can change even when you are not currently travelling. This is why a visual calendar and simulator are helpful.
Important: This article is educational only. It is not legal advice, immigration advice, or official government guidance. Always verify your own status, documents and itinerary with official sources or qualified professionals.