The European Commission says short stays in the Schengen Area are generally limited to a maximum of 90 days in any 180-day period. The key phrase is any 180-day period.
Why the window rolls
On each day of presence, look back across that day and the previous 179 days. Count every day spent in the Schengen Area. If the count exceeds 90, the general rule is breached.
Entry and exit days count
A stay beginning on 1 June and ending on 10 June uses ten days. The calculation is inclusive at both ends.
What should not be entered
The Commission says periods covered by an EU residence permit or long-stay visa are not subject to the normal 90/180 short-stay calculation. A short-stay visa can also authorise fewer than 90 days, in which case the visa itself controls.
Bilateral arrangements
The Commission's own guidance notes that certain older bilateral visa-waiver agreements can create more favourable treatment in specific states and are not handled by the general algorithm. ExpatEngine likewise does not attempt to model them.