I see. So this is actually what I wanted then and it seems to work in an intital test. Somehow this was easier than expected.
@BLUEFROG: You mentioned that this kind of text conversion needs scripting, but it seems that even if the document has the date in another format, like DD.MM.YY, the following document date placeholder action can be set to change that. That’s pretty cool.
This is my smart rule now:
Everyone, thanks again for the help!