Hi all, I'm George, the creator of the Schengen Simple app. Please feel free to shoot me any questions you might have.
I thought I’d explain a little more about how Schengen Simple works and why the 90/180 rule is deceptively complex.
The key thing to understand is that there is no beginning or end of the rolling 180-day window, so there isn’t a point where your allowance ‘resets’. For each and every day, there is a window of 180 days looking backwards - this is what makes the window "rolling"; it moves each day and has no beginning or end. The rule is you cannot exceed 90 days in ANY 180-day period. It's the "any" that makes things complicated, particularly if you want to maximise your allowance and have trips planned in the future.
This means any individual day of a trip will actually need to be counted in many 180-day windows - not just the 180-day window looking backwards. Schengen Simple instantly checks hundreds of thousands of these windows each time you save a trip, ensuring you can always safely travel for as long as possible.
This is quite different from other services, including the EU calculator, which simply tally up the days spent in the previous 180 days from the date they are checking. That approach will tell you “IF” you’ve overstayed (much like what a border official would check), but it can't tell you when you will overstay - it is not predictive. This approach can’t be used for planning purposes, as depending on the circumstance, it will both underestimate and, more consequently, overestimate your allowance (for one, it ignores your future plans).
