This Calendar Seminar was built many, many years ago, before I was using the ISO date standard. I'm going to guess it's probably a date formatting issue with the fact that you're using periods. Just to test it, try converting it over to a USA standard date format: 10/23/2000.

Thanks for your fast reply. Indeed, I thought about that as well, but I did not dare to switch my system to USA. So now after your answer I changed to USA temporarely and it works! Now I have to figure out, how to make it work with the Swiss or ISO date format.

Back then, when I made this, I used to be a huge fan of the Format function, so I could format dates in whatever style I wanted. I wasn't too worried about the fact that it converts the date to text. That should be a clue. You should be able to go through and just look for anything that relies on the actual text format. Instead of using that, like I often relied on the left two characters being the month. Just switch that to the actual month function, and it shouldn't be too hard to figure out.

I've got a version 2.0 of this calendar template that I'm getting ready to release soon. I think I'm pretty sure I went through and fixed all that stuff in it, and it didn't take me very long.