Hi all,
I decided to use the Templater Plug-in and found a good solution for me (see screen shots):
So thanks a lot everybody for your help.
PS: @anon12638239, if I put the template for the modification date in the frontmatter, it will not be updated correctly. Therefore I use a private block with a small table right below the frontmatter block. I think it looks pretty nice especially because I use the cssClass “table-tiny” in the frontmatter for all my tables.