Thanks for the heads up. The issue is that I was using AddElementProperty instead of AddComponentProperty.
Now the Date is displayed in the Designer of the Widget.
But got another issue now. When I hit Save in the Designer of the Widget Nothing happens and I get a js error.
Uncaught TypeError: Cannot read property 'toLowerCase' of undefined localhost:20593/ScriptResource.axd?d=ut6cYKafol9wWGp3sTJjPxiVlWuTuFB2b62RJz…S-GTV-MtnHQGQQj9K-xUrwFMXcgmgypJx-RgE6nsI_CAchXqC9vdd3Ido4ce0&t=43433ab4:2
When I changed it back to AddElementProperty in C# the error vanished
What do you think the issue is?