When entering a date with the date picker, the date is not correctly stored. 12. September 2017 will be saved as 9. Dezember 2017 13. September 2017 will not be saved, even if a date is required, because there is no 13. month. The date picker is presenting the selected date correctly, until it is saved.
DNN 9.1.1 german language pack as default 2sxc 9.4.1
The problem was already well known and seems to be solved in newer releases: https://github.com/2sic/2sxc/issues/1263