Great question — answering as the UphillTech team:
The Schengen Borders code was amended several time to make this clear. The current English-language wording is For intended stays on the territory of the Member States of a duration of no more than 90 days in any 180-day period, which entails considering the 180-day period preceding each day of stay You have to consider each day in turn, one-by-one. The number of days available on February 15 doesn't matter as such, you only need one. Then on February 16, you consider the preceding 180-day period (which is off by one day compared to the period you looked at on February 15) and you see if you still have one day left, etc. Incidentally, the main risk is not an overstay or fine but being...
Full answer by Relaxed: https://travel.stackexchange.com/a/193205 (CC BY-SA 4.0).
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