It may be a software issue with the BeoVision10. Usually you set the clock on the video master, in this case it isn't possible with the BeoVision10.
Remove the Masterlink from the BeoVision10 - option programme the BeoMaster7000 to option 1. Then set the clock as before. Once you have the time displayed on the BeoLab3500, revert back to your current Masterlink set up.
Then next summer do it all again. 