I have now done a few attempts to get the date format changed. I did, believing that deinstalling and reinstalling LR would change the time format to what it is at the computer. But it does not. There must be some information reminiscent from the old installation somewhere. This is, however, what I have done:
I first deinstalled LR, restarted the computer after a long time ago having changed the time format to 24 hours. When I started up LR again after a reinstallation, it came up with the last catalog and the pictures in it. And it still had the 12 hours format. I made a new catalog as a test and imported a picture from ViewNX2 (Nikons own editing program) that I knew had the 24 hour format. Imported into LR it comes up with 12 hours format. So, information about the format must be stored somewhere in supporting LR files. Therefore, after deinstalling LR, I deleted all catalog files and picture files in my Picture folder, deleted all support files in the Adobe folder in Library and the same with icons of LR in the Application folder. With Spotlight I searched for files with .lr extensions, having them suspicious of containing information about time formats. Finally I emptied the trash and restarted the computer (Powerbook Pro with ElCapital, X11.1.1).
Reinstalling LR took longer than last time, creating some hope. Opening it it comes up with no previous catalog , also promising. I imported one picture from my Nikon source that has the 24 hour format, and the time format is changed into 12 hours when the picture is "framed" by LR6. I can also see that LR somehow has found my Identity Plate somewhere, indicating that LR has stored some information somewhere that it uses during the installation process, that I cannot find, and where most likely the old information about the time format also is stored. I am sure that there is a file containing this information, that can be changed, if I knew where to find it and what to do with it. Reinstalling does not seems to help. I would be thankful for help to get this problem fixed.