Great, but...
I would honestly pay good money for a reliable app that offered the following:
1. A strong sense that I can trust it without having to double- and triple-check everything for possible errors
2. Good visual feedback in real time as changes are made to the dates of proposed future trips, as offered by the web browser version
3. Clear explanations of why the calculations are as they are. One of the hardest things about dealing with Schengen is the apparent lack of logic in it. It's so complicated to work out that it's almost impossible to be confident of any information offered by any app. That's why I'm using three separate calculators, just to be sure they all agree. The consequences of getting it wrong are potentially so severe (i.e. being banned for a long time from the Schengen area if one overstays) that one has to have total confidence in any app. Clear explanations of how the calculations are made, along with a total absence of bugs and errors, would really help in this regard.
This app gets close, but I'm still looking for the ultimate Schngen app that tells me what I need to know, helps me plan my future trips, and, most importantly, that I can trust 100%.