I just realized that I forgot to change the timezone/time settings in my camera since I am back from Asia (~6month). In order to fix the timestamp on all my photos I made during this time period I used exiftool for doing this on the command-line.
Move all timestamps minus 6 hours for all picture files (JPG and RAW) in current directory.
Check if everything is like expected and delete the backup copies in current directory:
Because I am working with Corel AfterShot (former Bibble) I had another problem. AfterShot is dumping the timestamps for all adjusted files into the xmp file and is also using these timestamps as first choice for displaying inside the application. But I figured out, that this information in not required and I just deleted it from the xmp file with sed.
Delete timestamp lines from mentioned files for all xmp files in current directory:
Actually a simple operation, but maybe helpful for someone else.