Thank you for your replies.
I have found the problem I had with displaying the Summary Levels. I had forgotten that I used some fonts which I had placed in the Fonts folder of the User, as opposed to System Fonts. For some reason these were not automatically migrated when I transferred my Yosemite files to Sierra. So this is NOT a Pan6 problem.
However, the date format problem most definitely is. I wrote to Jim Rea and he responded, as always, very quickly as follows:
Me: Do you have any bright ideas, please, how to force the date format I want?
Jim: Sorry, no, I don't. It will work in Panorama X, though, and you shouldn't even have to set the output pattern.
Me: Sierra doesn’t appear to have this problem with any other app.
Jim: At this point, Panorama 6 is probably the only application you have that still uses the old Carbon API's (the API's brought over from Mac OS 9).
SO, Jim is suggesting that changes made by Apple in Sierra account for the non-US date problem. No fix in sight.
I suppose the only workaround is to laboriously go through every database and change the dates manually to Text and enter them all again one by one. Not really an answer when you have over 100 databases going back to the OverVUE days!