Found out that the exif timestamps of my last holiday pictures had the wrong timezone. This can be easily corrected with Phil Harveys exiftool. There is even a description of exactly that problem in the documentation
In my case I did not only correct the timezone shift, but also the error to UTC time. For this purpose I simply took a picture of my GPS receivers clock display to compare the GPS / UTC timestamp to the EXIF data. It was a time shift of plus 59 minutes and 39 seconds so I used the following command line:
$ exiftool -AllDates+=:59:39 pictureDirectory
Note that by default exiftool generates backup copies to *_original files which you may want to delete afterwards.