Known issue with Residency Calculator.
This issue seems to be a problem only with Android 5.1.1 Lollipop.
If you change an entry/exit date that is not the last date, there is a maximum date limit that you can change the entry/exit date to. This maximum date limit is the day just before the next entry/exit date. The issue is, the maximum date limit date cannot be picked when in calendar mode.
The easiest way around this issue is to use spinner for date selection.
I have tested this issue on Android 6.0 Marshmallow, 7.0 Nougat and 8.1 Oreo, and it works fine. I guess it is an Android calendar date picking bug.