I set up a publishing project including a custom postamble :html-postamble
"Date: %d, Last modification: %C". When I export this file The %d date is
empty. From `C-h v org-html-postamble-format` I got the info about the
available elements to use:

```
  %t stands for the title.
  %s stands for the subtitle.
  %a stands for the author's name.
  %e stands for the author's email.
  %d stands for the date.
  %c will be replaced by `org-html-creator-string'.
  %v will be replaced by `org-html-validation-link'.
  %T will be replaced by the export time.
  %C will be replaced by the last modification time.
```

I tried to set both, :html-postamble and :html-postamble-format. Which date
is meant with the %d variable? Do I've to set it manually? I thought %d
will output the creation date of the file itself and %C the last updated
time (btw I get german weekdays and idk why & how to change).