Effective February 1st sunrise and sunset of Kassel will be used to define the night time for flight operations at night in Germany.
So far it is calculated by specified deviations for the longitudes of the place to be determined and in each case 30 minutes after sunset and 30 minutes before sunrise. So it could happen that even within a FIR different night times were used.
This calculation won't be necessary any more effective February 1st. Just look up the end of civil evening twilight (ECET) and the beginning of civil morning twilight (BCMT) in the respective table in the MIL AIP Germany.