How the 90/180 rule works
Non-EU nationals without a long-stay visa may spend a maximum of 90 days within any rolling 180-day period in the Schengen Area. The window is not a fixed calendar period — it rolls back 180 days from each day you are present. This means you cannot simply reset the clock on a fixed date.
Both the day of entry and the day of departure count as days of presence. Transit through a Schengen airport without passing border control does not count.