This single-file solution provides a robust, professional-grade Schengen Visa calculator. It manages state in memory, adheres to the 90/180-day sliding window rule, and provides a polished, responsive user interface. ```html
Track your 90/180 day allowance.
The 90/180-day rule states that for any given day, your total stay in the Schengen Area in the preceding 180 days must not exceed 90 days.
1. Add your past and planned trips.
2. The calculator automatically analyzes every day in your timeline.
3. The Dashboard shows if you are compliant or at risk of an overstay.