Huge respect to the author for the details that have gone into this. I'd spent a week hammering at a Claude max 20x plan to try and build schengen 90/180 rolling window + tax residency in a couple of countries tracker... and that was hard work. I can only imagine how much effort has gone into this, to get all the details right.

It's unclear whether the author wrote all of this themselves, or if they outsourced a bunch of it to Claude. My experience with Claude was that it was terrible at writing code to do the math, even when I explained what the calculation needed to be, what the input was, and what the expected result was. It ultimately took starting a whole new project just to do the rolling window calculation, and then have that fed back in.

My biggest question for the author, if they happen to see this, is: how much manual testing validation did you do of the outputs the app produces? IE: Did you do the inputs + transformations = output calculations yourself as well, counting days on calendars, etc, to validate that the app is actually accurate? (That was the only way I developed any faith in solution I made for myself, which is way less impressive than your app). Regardless of whether you wrote the code yourself or not, a thorough test harness feels vitally important for an app like this.