When does the 180-day period start in Schengen?

The window counts backwards, not forwards

The natural way to read "90 days in a 180-day period" is forwards: something starts a clock, and 180 days later the clock ends. The regulation works the other way around. For each day of your presence, the relevant 180-day period is the one ending on that day — the day itself plus the 179 days before it. Compliance is checked against that backwards-looking window, and the check repeats every day, with the window sliding one day forward each time.

So the honest answer to "when does it start?" is: 179 days before whatever day you're asking about. Ask on a different day, get a different window.

What the window is NOT anchored to

• Not your first entry. Your first-ever arrival in Schengen does not open a personal 180-day season. Days before it simply count as zero.
• Not your visa. For visa holders, the visa validity window is a separate, additional constraint — it does not define the 180-day period.
• Not the calendar. There are no half-year blocks (January–June, July–December) and nothing special happens at the new year.
• Not your last exit. Leaving doesn't close a period or open a new one — the window keeps sliding whether you are inside or outside the area.

A concrete example

You plan to enter on October 10. The window that matters for that day runs from April 14 to October 10 — count every Schengen day in that range, and you may enter if the total (including October 10 itself) stays at or below 90. If you then remain until October 20, the window for October 20 runs April 24 – October 20: ten days shifted, possibly with old spring days having dropped off the back. Each day of your stay must pass its own check — that's why a stay that starts legally can still become an overstay partway through if old days aren't aging out fast enough.

Counting 180-day ranges by hand is error-prone, which is why the European Commission publishes a calculator and why we built a friendlier one: the Schengen 90/180 calculator checks every day of a planned stay at once and flags the first day that would break the rule.

Sources

• Regulation (EU) 2016/399 (Schengen Borders Code), Article 6 — entry conditions
• European Commission — Short-stay visa calculator