Counting Schengen days sounds like a job for a spreadsheet until you are standing at a border trying to remember whether that long weekend in March counts. It does. They all count. That is why a mobile app for counting Schengen days exists, and why picking the right one matters more than it looks.
Since the names in this niche change faster than the rules, here is a criteria-based comparison instead of a ranking that would be stale by spring.
First filter: the math engine. The app must compute the rolling 180-day window correctly. For any given date: how many of the previous 180 days were spent inside the zone. Apps that only track calendar years, or that count trips instead of days, are wrong in ways that create overstays. Test any candidate against dates you already know.
Second filter: the country list. Schengen membership is not the same as EU membership, and it changes. A good app maintains an accurate, checkable list of which countries count toward your 90 days. If the app cannot show you its list, or the list looks frozen in time, move on.
Third filter: data entry you will actually use. The best apps let you log a trip in seconds and fix mistakes without starting over. Import from your calendar or TripIt is a nice bonus, but manual entry that takes ten seconds beats a clever import you never set up.
Fourth: offline mode, non-negotiable. You check this number at borders and on planes, not on home Wi-Fi. And the basic counter should be free. This is arithmetic. A Schengen 90/180 calculator app free tier is normally the complete product.
What no counter does is verify the rules behind your count. Run the Schengen 90/180 day calculator search, pick the app that passes the filters above, and feed it honest dates from your passport stamps.
Then keep the rules current too. The passport-plus-destination eligibility report at visadocs.fyi gives you the entry picture for your exact passport and destination, so your careful counting is built on current information.